pgsql: Rewrite comment code for better modularity, and add necessary

[rhaas(at)postgresql(dot)org (Robert Haas)]

Log Message:
-----------
Rewrite comment code for better modularity, and add necessary locking.

Review by Alvaro Herrera, KaiGai Kohei, and Tom Lane.

Modified Files:
--------------
    pgsql/src/backend/catalog:
        Makefile (r1.78 -> r1.79)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/Makefile?r1=1.78&r2=1.79)
    pgsql/src/backend/commands:
        comment.c (r1.117 -> r1.118)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/comment.c?r1=1.117&r2=1.118)
    pgsql/src/include/catalog:
        dependency.h (r1.45 -> r1.46)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/dependency.h?r1=1.45&r2=1.46)

Added Files:
-----------
    pgsql/src/backend/catalog:
        objectaddress.c (r1.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/objectaddress.c?rev=1.1&content-type=text/x-cvsweb-markup)
    pgsql/src/include/catalog:
        objectaddress.h (r1.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/objectaddress.h?rev=1.1&content-type=text/x-cvsweb-markup)