xlocale(3): Link man pages

• provide various missing MLINKS for library functions
• update various SEE ALSO section to include the new linked manual pages
• add various definitions of new functions like isideogram_l(3)
• document COMPATIBILITY for some functions
• bump man page dates

Reviewed by: gbe, bcr
MFC after: 1 week
Pull Request: https://github.com/freebsd/freebsd-src/pull/621
Differential Revision: https://reviews.freebsd.org/D37203