pgsql: Repair an error introduced by log_line_prefix patch: it is not

Log Message:
-----------
Repair an error introduced by log_line_prefix patch: it is not acceptable
to assume that the string pointer passed to set_ps_display is good forever.
There's no need to anyway since ps_status.c itself saves the string, and
we already had an API (get_ps_display) to return it.
I believe this explains Jim Nasby's report of intermittent crashes in
elog.c when %i format code is in use in log_line_prefix.
While at it, repair a previously unnoticed problem: on some platforms such as
Darwin, the string returned by get_ps_display was blank-padded to the maximum
length, meaning that lock.c's attempt to append " waiting" to it never worked.

Modified Files:
--------------
    pgsql/src/backend/postmaster:
        postmaster.c (r1.474 -> r1.475)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/postmaster.c.diff?r1=1.474&r2=1.475)
    pgsql/src/backend/storage/lmgr:
        lock.c (r1.158 -> r1.159)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/lock.c.diff?r1=1.158&r2=1.159)
    pgsql/src/backend/utils/error:
        elog.c (r1.166 -> r1.167)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/error/elog.c.diff?r1=1.166&r2=1.167)
    pgsql/src/backend/utils/misc:
        ps_status.c (r1.25 -> r1.26)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/ps_status.c.diff?r1=1.25&r2=1.26)
    pgsql/src/include/libpq:
        libpq-be.h (r1.52 -> r1.53)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/libpq/libpq-be.h.diff?r1=1.52&r2=1.53)
    pgsql/src/include/utils:
        ps_status.h (r1.25 -> r1.26)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/ps_status.h.diff?r1=1.25&r2=1.26)