erlang: Add manpages to all release packages.

This puts manpages into "$out/lib/erlang/man" and also symlinks them
into "$out/share/man" with an "erl" suffix (similar to how Debian is
handling this) so those manpages are unlikely to collide with any other
manpages, like for example inet(3).

Closes #3396.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Acked-by: Moritz Ulrich <moritz@tarn-vedra.de>