warning C4706: assignment within conditional expression

Rather than use TRUE, FALSE, NULL, 0 or != 0 in if/while conditions.

Additionally, corrected some example code to adhere to the recommended
coding style.

warning: conversion to 'unsigned char' from 'int' may alter its value

vtls/mbedtls.h:67:36: warning: implicit declaration of function
‘mbedtls_sha256’ [-Wimplicit-function-declaration]

Following commit c5744340. Additionally removes the need for a second
'result code' variable as well.

.. because curl_multi_wait can no longer signal poll failure.

follow-up to 77e17267

Bug: https://github.com/curl/curl/issues/707

As these files don't need to contain references to the source files,
although typically do, added basic files which only include three
filters and don't require the project file generator to be modified.

These files allow the source code to be viewed in the Solution Explorer
in versions of Visual Studio from 2010 onwards in the same manner as
previous versions did rather than one large view of files.

warning C4706: assignment within conditional expression

warning C4668: 'USE_IPV6' is not defined as a preprocessor macro,
               replacing with '0' for '#if/#elif'

warning C4701: potentially uninitialized local variable 'size' used

Technically this can't happen, as the usage of 'size' is protected by
'if(parsed)' and 'parsed' is only set after 'size' has been parsed.

Anyway, lets keep the compiler happy.

Since gcc 5, the processor output can get split up on multiple lines
that made the configure script fail to figure out values from
definitions. The fix is to use cpp -P, and this fix now first checks if
cpp -P is necessary and then if cpp -P works before it uses that to
extract defined values.

Fixes #719

formdata.c:390: warning: cast from pointer to integer of different size

Introduced in commit ca5f9341 this happens because a char*, which is
32-bits wide in 32-bit land, is being cast to a curl_off_t which is
64-bits wide where 64-bit integers are supported by the compiler.

This doesn't happen in 64-bit land as a pointer is the same size as a
curl_off_t.

This fix doesn't address the fact that a 64-bit value cannot be used
for CURLFORM_CONTENTLEN when set in a form array and compiled on a
32-bit platforms, it does at least suppress the compilation warning.

The OpenSSL API change that broke this is "Convert ERR_STATE to new
multi-threading API": openssl commit 8509dcc.

Closes #713

follow-up to 80015cdd

... to allow users to see which specfic wildcard that matched when such
is used.

Also minor logic cleanup to simplify the code, and I removed all tabs
from verbose strings.

It would also seem that share.h is not required here either as there
are no references to the Curl_share structure or functions.

Bug: https://curl.haxx.se/mail/lib-2016-03/0150.html
Reported-by: Oliver Graute

... to allow torture tests then too.

As just discussed on the mailing list, also document how we prefer
spacing in expressions.

cppcheck warned:

[src/tool_urlglob.c:283]: (style) Checking if unsigned variable 'step_n'
is less than zero.

Ripped out from CONTRIBUTE into its own document, but also extended from
there.

Since sh_getentry() now checks for invalid sockets itself and by
narrowing the scope of the remove_sock_from_hash variable.

Simplify the code by using a single entry that looks for a socket in the
socket hash. As indicated in #712, the code looked for CURL_SOCKET_BAD
at some point and that is ineffective/wrong and this makes it easier to
avoid that.

Closes #712