build-system: update for split man pages
- We have to remove the installation of notmuch.1.gz from the top
level Makefile.local.
- Man pages with multiple names are handled by making relative
symlinks in the install-man target.
- update version tests and convenience rules for split man pages
The man page version test still only checks notmuch.1, but the
location is updated.
update-man-versions is longer than the one-line previously in
update-versions mainly because I decided to take the high road and
stick to POSIX sed (thus, no sed -i). The sed regex itself is more
complicated to cope with variations in the headers.