Namespace clean-up for time64.

Namespace clean-up for time64.

All of the time64 functions are only visible within pp_sys.c and are not
part of the public API, so make them all static and give them the S_ prefix.
A side effect of this is that we now unambiguously use S_localtime64_r and
S_gmtime64_r and should no longer collide with system-supplied versions, if
any.

One possible future direction is that if and when Configure detects any
system-supplied *time64_r functions, their prototypes, and their valid
time ranges, we could optionally select those instead of the home-grown
ones.

Another possible future direction is that we could s/S_/Perl_/ and make
the *time64_r functions part of the public API in some post-5.10.x
release.  Currently they are only exposed to the outside world via
Perl_pp_gmtime.

Also gave some TRACE macros more specific names.