Fcntl, POSIX and Socket can all use Proxy Constant Subs unconditionally.

Fcntl, POSIX and Socket can all use Proxy Constant Subs unconditionally.

Previously each Makefile.PL had conditional code to only use them post 5.9.2,
so that the code would not diverge between blead and maint-5.8. That isn't an
important consideration any longer, whereas removing the conditional code here
will allow their AUTOLOAD routines to be simplified.