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

... to let tests require it or skip if present

Make the name reflect its use better, and add a short comment describing
what it's for.

... and make sure inet_pton is always checked for when *not* using Windows,
which is a regression from 4fc6ebe1.

Idea-by: Sergei Nikulov

... by checking the POLLIN define, as the header file checks don't work
on Windows.

inet_pton() exists on Windows and gets used by our cmake builds. Make
sure the correct header file is included to avoid compiler warnings.

Closes #1639

Closes #1572

... by simply trying the Windows argument types first.

Fixes #1640