foo_cloexec() under PERL_GLOBAL_STRUCT_PRIVATE

foo_cloexec() under PERL_GLOBAL_STRUCT_PRIVATE

Fix the various Perl_PerlSock_dup2_cloexec() type functions so that
t/porting/liberl.a passes under -DPERL_GLOBAL_STRUCT_PRIVATE builds.

In these builds it is forbidden to have any static variables, but each
of these functions (via convoluted macros) has a static var called
'strategy' which records, for each function, whether a run-time probe
has been done to determine the best way of achieving close-exec
functionality, and the result.

Replace them all with 'global' vars: PL_strategy_dup2 etc.

NB these vars aren't thread-safe but it doesn't really matter, as the
worst that can happen is for a redundant probe or two to be done before
a suitable "don't probe any more" value is written to the var and seen
by all the threads.