Better skipping of tests requiring fork()

Windows doesn't have a real fork() and only has a fork() emulation when
built with ithreads and -D PERL_IMPLICIT_SYS. ($Config{d_pseudofork} can
be used to identify such builds on recent perls, but was only added in
Perl 5.8.9/5.10.0.)
Merge pull request #18 from steve-m-hay/master

Better skipping of tests requiring fork(), especially when used on windows with non-default configuration