Add 'locale' warning category

This category will be used in future commits for warnings that are
entirely because of locale issues.
perldiag: Remove message that is not raised

This message isn't raised these days.
Raise warnings for poorly supported locales

Perl only supports single-byte locales (except for UTF-8 ones), and has
poor support for 7-bit locales that aren't supersets of ASCII (these
should be exceedingly rare these days).

This commit raises warnings in the new locale warning category when
such a locale is entered.
lib/locale.t: Add knowledge about known bad locales

Some platforms currently have locale tests that always fail due to
platform-specific problems.  For example, we know that some Lithuanian
locales fail on Darwin because of a bad symbolic link there.

This commit adds the capability for locale.t to know about such issues
and not fail because of them.
perllocale: Nits and clarifications