charsets.7: Update to reflect past developments

Rewrite the introduction to make Unicode's prominence more obvious.
Reformulate parts of the text to reflect current Unicode world.
Minor clarification for ASCII/ISO sections, some other minor fixes.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
charsets.7: Minor tweaks

And restore a piece about Biblical Hebrew that was
inadvertently deleted by Marko Myllynen's patch.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
repertoiremap.5: New page for repertoiremap(5)

Rather obsolete feature but localedef(1) refers to repertoiremaps.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
charmap.5: Update to match current glibc

charmap(5) was outdated, bring it to closer to reality by fixing
syntax descriptions to match current glibc code and practices,
adding missing options, removing obsolete comments and references,
and removing now incorrect examples.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
localedef.1: Align with recent charmap(5) / repertoiremap(5) changes

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>