pgsql: Code cleanup: mark some variables with the "const" modifier, when

Log Message:
-----------
Code cleanup: mark some variables with the "const" modifier, when they
are initialized with a string literal. Patch from Stefan Huehner.

Modified Files:
--------------
    pgsql/src/backend/utils/adt:
        ruleutils.c (r1.255 -> r1.256)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/ruleutils.c.diff?r1=1.255&r2=1.256)
    pgsql/src/bin/initdb:
        initdb.c (r1.134 -> r1.135)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/initdb/initdb.c.diff?r1=1.134&r2=1.135)
    pgsql/src/bin/pg_controldata:
        pg_controldata.c (r1.33 -> r1.34)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_controldata/pg_controldata.c.diff?r1=1.33&r2=1.34)
    pgsql/src/bin/pg_ctl:
        pg_ctl.c (r1.78 -> r1.79)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_ctl/pg_ctl.c.diff?r1=1.78&r2=1.79)
    pgsql/src/bin/pg_dump:
        pg_backup_archiver.c (r1.142 -> r1.143)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_backup_archiver.c.diff?r1=1.142&r2=1.143)
        pg_backup_custom.c (r1.37 -> r1.38)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_backup_custom.c.diff?r1=1.37&r2=1.38)
        pg_backup_files.c (r1.31 -> r1.32)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_backup_files.c.diff?r1=1.31&r2=1.32)
        pg_backup_tar.c (r1.57 -> r1.58)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_backup_tar.c.diff?r1=1.57&r2=1.58)
        pg_dump_sort.c (r1.17 -> r1.18)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_dump/pg_dump_sort.c.diff?r1=1.17&r2=1.18)
    pgsql/src/bin/psql:
        describe.c (r1.153 -> r1.154)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/describe.c.diff?r1=1.153&r2=1.154)