MSVC warns that gethostbyname is deprecated. Always use getaddrinfo
instead to fix this when IPv6 is enabled, also for IPv4 resolves. This
is also consistent with what libcurl does.

Closes https://github.com/curl/curl/pull/1682

- s/SessionHandle/Curl_easy/

Bug: https://github.com/curl/curl/commit/eb16305#commitcomment-23035670
Reported-by: Gisle Vanem

Older GCC warns:
/tests/server/rtspd.c:1194:10: warning: missing braces around
initializer [-Wmissing-braces]

Fix this by using memset instead of an initializer.

With warning level 4, MSVC warns about assignments within conditional
expressions. Change the while loop to a do-while loop to fix this. This
change is also consistent with CODE_STYLE.md.

MSVC warns when implicitly casting -1 to unsigned long.

warning C4701: potentially uninitialized local variable 'req' used

Disabled in cda19a34. It only needs to
be disabled for libcurl.

This is consistent with 7bc64561, which
changed the warning level from 3 to 4 for the Visual Studio project
files. But disable the level 4 warning C4127 "conditional expression is
constant", as that one is issued by older versions of the Windows SDK
as well as curl itself under some circumstances.

Closes https://github.com/curl/curl/pull/1667

Install libidn2 to increase test coverage (IDN tests)

Closes https://github.com/curl/curl/pull/1673

... to get warnings also on Linux/GCC and OSX/clang.

Closes https://github.com/curl/curl/pull/1666

Install libssh2 to increase test coverage (SFTP, SCP)

... to avoid compiler warnings if the user doesn't want
WIN32_LEAN_AND_MEAN.

It's defined for all build systems in curl_setup.h since commit
beb08481. This caused macro
redefinition warnings in the configure builds.

Closes https://github.com/curl/curl/pull/1677

Make sure to always define WIN32_LEAN_AND_MEAN before including any
Windows headers to avoid pulling in unnecessary headers. This avoids
unnecessary macro clashes and compiler warnings.

Ref: https://github.com/curl/curl/issues/1562
Closes https://github.com/curl/curl/pull/1672

This is a follow-up to af021625 which removed (SET_)ERRNO macros. That
commit was an earlier draft that I committed by mistake, which was then
remedied by a5834e52 and e909de65, and now this commit. With this commit
there is now no difference between the current code and the changes that
were approved in the final draft.

Thanks-to: Max Dymond, Marcel Raad, Daniel Stenberg, Gisle Vanem
Ref: https://github.com/curl/curl/pull/1589

Closes https://github.com/curl/curl/pull/1671

Ref: #1591

ldap_bind_s is marked as deprecated in w32api's winldap.h shipping with
the latest original MinGW, resulting in compiler warnings since commit
f0fe66f1. Fix this for the non-SSPI
case by using ldap_simple_bind_s again instead of ldap_bind_s with
LDAP_AUTH_SIMPLE.

Closes https://github.com/curl/curl/pull/1664

When building with Cygwin or MinGW, libtool uses a wrapper executable
instead of a wrapper script [1], which is written in C and throws
missing-variable-declarations warnings. Don't enable these warnings on
Cygwin and MinGW in order to avoid warnings for every executable built,
which spams the test suite output when using Cygwin's clang.

[1] https://www.gnu.org/software/libtool/manual/html_node/Wrapper-executables.html

Closes https://github.com/curl/curl/pull/1665

Prior to this change (SET_)ERRNO mapped to GetLastError/SetLastError
for Win32 and regular errno otherwise.

I reviewed the code and found no justifiable reason for conflating errno
on WIN32 with GetLastError/SetLastError. All Win32 CRTs support errno,
and any Win32 multithreaded CRT supports thread-local errno.

Fixes https://github.com/curl/curl/issues/895
Closes https://github.com/curl/curl/pull/1589

GCC 4.6.3 on travis complains:
smb.c: In function ‘get_posix_time’:
smb.c:725:13: error: declaration of ‘time’ shadows a global declaration
[-Werror=shadow]

Fix this by renaming the variable.

GCC 4.4 complains:
tool_cb_wrt.c:81: error: declaration of ‘isatty’ shadows a global
declaration
/usr/include/unistd.h:782: error: shadowed declaration is here

Fix this by renaming the variable.

Closes https://github.com/curl/curl/pull/1661

Closes #1653

Removes BUILD_RELEASE_DEBUG_DIRS since it wasn't used anywhere.

Closes #1649

Add a basic telnet server for negotiating some telnet options before
echoing back any data that's sent to it.

Closes #1645

I added a selection of torture and event tests that run "fast enough"

Fixes #1612
Reported-by: Jeroen Ooms

Bug: https://github.com/curl/curl/issues/828#issuecomment-313475151

Fixes #1552