Add a deprecated attribute to functions and enum values that should not
be used anymore.
This uses a gcc 4.3 dialect, thus is only available for this version of
gcc and newer. Note that the _Pragma() keyword is introduced by C99, but
is available as part of the gcc dialect even when compiling in C89 mode.
It is still possible to disable deprecation at a calling module compile
time by defining CURL_DISABLE_DEPRECATION.
Gcc type checking macros are made aware of possible deprecations.
Some testing support Perl programs are adapted to the extended
declaration syntax.
Several test and unit test C programs intentionally use deprecated
functions/options and are annotated to not generate a warning.
New test 1222 checks the deprecation status in doc and header files.
Closes#9667
