Install section 3 manpages to .../share/man for consistency

When an admin installs git site-wide, e.g. to /usr/local, perl's makemaker
decides to use 'site' mode (as opposed to distro or user mode). For whatever
reason, it then installs all perl-related manpages (those in section 3) to
.../man, while the rest of git happily installs manpages to .../share/man.

In as such, this is different from the patch proposed in this thread:
  http://marc.info/?t=117862141100003&r=1&w=2

My patch should only ever affect site installs and does not cause
installations to $HOME to go to $HOME/share/...