Add t/porting/perlfunc.t so that porting tests catch problems with perlfunc

Add t/porting/perlfunc.t so that porting tests catch problems with perlfunc

Pod::Functions is now generated from pod/perlfunc.pod by
ext/Pod-Functions/Functions_pm.PL

If it can't parse pod/perlfunc.pod, it will abort, which will cause the
build to break. It's really not possible for it to carry on, hence aborting
is the only option. However, innocent-seeming changes to documentation
shouldn't break the build, and we expect everyone to run (at least)
the porting tests, hence this test, to catch such problems before it's too
late. To avoid duplicating the parsing logic, we make Functions_pm.PL take
a --tap option, to test that all is well.