Arrange for the postmaster (and standalone backends, initdb, etc) to

Arrange for the postmaster (and standalone backends, initdb, etc) to
chdir into PGDATA and subsequently use relative paths instead of absolute
paths to access all files under PGDATA.  This seems to give a small
performance improvement, and it should make the system more robust
against naive DBAs doing things like moving a database directory that
has a live postmaster in it.  Per recent discussion.