make mercurial look in more places for config files.
now it searches <install dir>/etc/mercurial, /etc/mercurial, and user
hgrc.
this allows site-wide configuration to be shared over automounted nfs
partition, instead of chenging on every system. option of having local
configuration on every system remains.
old code for searching /etc/mercurial/hgrc.d never worked, this code
is tested and works.