Install man3 manpages to $PREFIX/share/man/man3 even for site installs

Install man3 manpages to $PREFIX/share/man/man3 even for site installs

MakeMaker supports three installation modes: perl, site, and vendor. The first
and third install manpages to $PREFIX/share/man, only site installs to
$PREFIX/man. For consistency with the rest of git, which does not make the
distinction and writes all manpages to $PREFIX/share/man, this change makes
sure that perl does too, even when it's installed in site mode.

Signed-off-by: martin f. krafft <madduck@madduck.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>