The current version of speedcheck.c may disable timeout by setting zero
to Curl_expire. Which is fine using the curl_multi_perform, because it
recheck all timeout internals, but when using custom event poller (like
hiperfifo.c) it may keep stalle connection forever.

Adjust tests/libtest/Makefile.inc and remove a couple of unused headers from
tests/libtest/lib583.c

Added missing memoryTracking to test cases 560 and 583. If this triggers
leak detection on these, it only means that previously it was going unnoticed.

No feedback and no updates, removing these issues

Calling sclose() both in the child and the parent fools the
socket leak detector into thinking it's been closed twice.
Calling close() in the child instead overcomes this problem. It's
not as portable as the sclose() macro, but this code is highly
POSIX-specific, anyway.

Just internal stuff...

Curl_safefree is now a macro defined in memdebug.h instead of a function
prototyped in url.h and implemented in url.c, so inclusion of url.h is no
longer required in order to simply use Curl_safefree.

Provide definition of macro WHILE_FALSE in setup_once.h in order to allow
other macros such as DEBUGF and DEBUGASSERT, and code using it, to compile
without 'conditional expression is constant' warnings.

The WHILE_FALSE stuff fixes 150+ MSVC compiler warnings.

Ensure existing logic in Curl_resolv_timeout() is not subverted upon getting a
negative timeout from resolve_server(). The timeout in resolve_server() could
be checked to avoid calling Curl_resolv_timeout() with an expired timeout, but
fixing this in this way allows existing logic in resolve_server() to be kept
unchanged.

follow-up for commit d20408e8

Regenerate curl's tests client knownhosts file also when, somehow,
this file is empty.

Regenerate curl's tests host and client key files also when, somehow,
any of these files are empty.

follow-up for commit c06de200