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.