libc: Move NLS catalogues to the locales packages

We don't want to put these in clibs (where libc is) since they are not
critical to system operation. Move them to locales, since anyone who
is interested in translated versions of strerror() is going to have
that installed anyway.

While here, add some more documentation to bsd.nls.mk, particularly
the NLSPACKAGE option.

MFC after: 3 seconds
Reviewed by: manu, kib
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D52898