pgsql: Add pg_postmaster_start_time() function.

Log Message:
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Add pg_postmaster_start_time() function.

Euler Taveira de Oliveira
Matthias Schmidt

Modified Files:
--------------
pgsql/doc/src/sgml:
func.sgml (r1.254 -> r1.255)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml.diff?r1=1.254&r2=1.255)
pgsql/src/backend/postmaster:
postmaster.c (r1.452 -> r1.453)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/postmaster.c.diff?r1=1.452&r2=1.453)
pgsql/src/backend/tcop:
postgres.c (r1.447 -> r1.448)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/postgres.c.diff?r1=1.447&r2=1.448)
pgsql/src/backend/utils/adt:
timestamp.c (r1.124 -> r1.125)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/timestamp.c.diff?r1=1.124&r2=1.125)
pgsql/src/include/catalog:
pg_proc.h (r1.366 -> r1.367)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h.diff?r1=1.366&r2=1.367)
pgsql/src/include/utils:
timestamp.h (r1.43 -> r1.44)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/timestamp.h.diff?r1=1.43&r2=1.44)