Remove all direct comparisons to 'NULL' for pointers.
Replace them with boolean-like 'if (ptr)' and 'if (!ptr)'.
This makes conditions shorter and easier to read.
Signed-off-by: Alexander Amelkin <alexander@amelkin.msk.ru>
There are lots of feature test macros like _POSIX_SOURCE,
_BSD_SOURCE, etc. chaotically placed at the beginning of
multiple files without apparent reason, sometime purely
erroneously.
This commit removes them all for the sake of cleaner code.
No new warnings are added due to that at least for gcc 5.4
on Linux with glibc 2.23.
Commit adds macros and #include in order to bring down the number of `warning:
implicit declaration of function 'X' [-Wimplicit-function-declaration]`
warnings.
