pgsql: Move session_start out of MyProcPort stucture and make it a

Log Message:
-----------
Move session_start out of MyProcPort stucture and make it a global called MyStartTime,
so that we will be able to create a cookie for all processes for CSVlogs.
It is set wherever MyProcPid is set. Take the opportunity to remove the now
unnecessary session-only restriction on the %s and %c escapes in log_line_prefix.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        config.sgml (r1.134 -> r1.135)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml.diff?r1=1.134&r2=1.135)
    pgsql/src/backend/bootstrap:
        bootstrap.c (r1.235 -> r1.236)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/bootstrap/bootstrap.c.diff?r1=1.235&r2=1.236)
    pgsql/src/backend/postmaster:
        autovacuum.c (r1.55 -> r1.56)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/autovacuum.c.diff?r1=1.55&r2=1.56)
        pgarch.c (r1.29 -> r1.30)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgarch.c.diff?r1=1.29&r2=1.30)
        pgstat.c (r1.161 -> r1.162)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgstat.c.diff?r1=1.161&r2=1.162)
        postmaster.c (r1.536 -> r1.537)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/postmaster.c.diff?r1=1.536&r2=1.537)
        syslogger.c (r1.34 -> r1.35)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/syslogger.c.diff?r1=1.34&r2=1.35)
    pgsql/src/backend/tcop:
        postgres.c (r1.536 -> r1.537)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/postgres.c.diff?r1=1.536&r2=1.537)
    pgsql/src/backend/utils/error:
        elog.c (r1.190 -> r1.191)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/error/elog.c.diff?r1=1.190&r2=1.191)
    pgsql/src/backend/utils/init:
        globals.c (r1.101 -> r1.102)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/init/globals.c.diff?r1=1.101&r2=1.102)
    pgsql/src/include:
        miscadmin.h (r1.195 -> r1.196)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/miscadmin.h.diff?r1=1.195&r2=1.196)
    pgsql/src/include/libpq:
        libpq-be.h (r1.62 -> r1.63)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/libpq/libpq-be.h.diff?r1=1.62&r2=1.63)