pgsql: pg_upgrade: remove pg_multixact files left by initdb

pg_upgrade:  remove pg_multixact files left by initdb

This fixes a bug that caused vacuum to fail when the '0000' files left
by initdb were accessed as part of vacuum's cleanup of old pg_multixact
files.

Backpatch through 9.3

Branch
------
REL9_4_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/dd5369047fc3400f4b5d1d61d85b040a8eef8d1b

Modified Files
--------------
contrib/pg_upgrade/pg_upgrade.c |   33 +++++++++++++++++++++++++++------
1 file changed, 27 insertions(+), 6 deletions(-)