Non-stub gettext API functions committed, ready for testing

[Rich Felker <dalias@libc.org>]

As of commit 2068b4e8911a3a49cded44b4568f6c943a8c98f8, it should now
be possible to support message translation at the application level
using the gettext (libintl.h) functions provided by musl. Feedback
from users interested in this functionality would be much appreciated!

Note that some (many?) applications may attempt to use their own
included gettext rather than the one in libc, so before testing it
would be helpful to check this and make sure the functions in musl are
actually getting called. I'm not familiar with what types of checks
typical autoconf scripts do to choose whether to use libc gettext or
their own, so information on this topic would also be helpful,
especially if there's anything we could do to get apps to choose the
one in musl rather than pulling in their own bloated copy of GNU
gettext.

As mentioned in the commit message, some functionality is still
missing. For the plurals stuff, I can't find the information on how
you actually get the plural rules out of the .mo file and apply them.
For the LANGUAGE variable, it's just a matter of adding some
loop-and-retry logic.

Rich