Windows doesn't have M_PI; define it ourselves when needed.

This should fix the Windows build, broken by commit ed802e7d.
pg_upgrade:  remove reference to autovacuum_multixact_freeze_max_age

autovacuum_multixact_freeze_max_age was added as a pg_ctl start
parameter in 9.3.X to prevent autovacuum from running.  However, only
some 9.3.X releases have autovacuum_multixact_freeze_max_age as it was
added in a minor PG 9.3 release.  It also isn't needed because -b turns
off autovacuum in 9.1+.

Without this fix, trying to upgrade from an early 9.3 release to 9.4
would fail.

Report by EDB

Backpatch through 9.3
pg_upgrade:  assume user is install user

The user specified to the upgrade was effectively the install user, but
that was not clearly stated in the comments, documentation, or error
messages.
Improve some JSON error messages.

These messages are new in 9.4, which hasn't been released yet, so
back-patch to REL9_4_STABLE.

Daniele Varrazzo
Fix typo in C comment.
Change ParseConfigFp() so that it doesn't process unused entry of each parameter.

When more than one setting entries of same parameter exist in the
configuration file, PostgreSQL uses only entry appearing last in
configuration file scan. Since the other entries are not used,
ParseConfigFp() doesn't need to process them, but previously it did
that. This problematic behavior caused the configuration file scan
to detect invalid settings of unused entries (e.g., existence of
multiple entries of PGC_POSTMASTER parameter) and log the messages
complaining about them.

This commit changes the configuration file scan so that it processes
only last entry of each parameter.

Note that when multiple entries of same parameter exist both in
postgresql.conf and postgresql.auto.conf, unused entries in
postgresql.conf are still processed only at postmaster startup.

The problem has existed since old version, but a user is more likely
to encounter it since 9.4 where ALTER SYSTEM command was introduced.
So back-patch to 9.4.

Amit Kapila, slightly modified by me. Per report from Christoph Berg.
Committing to epic branch
committing
Convert spaces to tabs
Initial ideas on how extracting the list of signatures could work
More formatting fixes
Make mdc_filter private again

pgp-info.c doesn't need to use it
Fix copypasteo
Fixes to pgp-info.c
I still don't like the comment
Reorganize code; create pgp-sig.c
Remove duplicated code
more cleanup
more cleanup
do stuff