portable Perl_my_mkostemp()

portable Perl_my_mkostemp()

Akin to the existing Perl_my_mkstemp(), Perl_my_mkostemp() is defined
as a macro for mkostemp() where available, and otherwise as our own
implementation of it.  The guts of our own implementations of the two
functions are shared.  Perl_my_mkostemp() is not guaranteed to handle
O_CLOEXEC: that depends on open() handling it.