pgsql: pg_upgrade: detect stale postmaster.pid lock files

pg_upgrade:  detect stale postmaster.pid lock files

If the postmaster.pid lock file exists, try starting/stopping the
cluster to check if the lock file is valid.

Per request from Tom.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/a9ceaa53be766a324e41953a78e8bd9fa408edf6

Modified Files
--------------
contrib/pg_upgrade/check.c      |   11 ++++-----
contrib/pg_upgrade/exec.c       |   10 ++-----
contrib/pg_upgrade/pg_upgrade.c |   48 +++++++++++++++++++++++++++++---------
contrib/pg_upgrade/pg_upgrade.h |    6 ++--
contrib/pg_upgrade/server.c     |    9 +++++-
5 files changed, 54 insertions(+), 30 deletions(-)