pgsql: Add pg_postmaster_start_time() function.

[momjian(at)svr1(dot)postgresql(dot)org (Bruce Momjian)]

Log Message:
-----------
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)