pgsql: Disallow changing/dropping default expression of a SERIAL column

Log Message:
-----------
Disallow changing/dropping default expression of a SERIAL column

Dhanaraj M

Modified Files:
--------------
    pgsql/src/backend/catalog:
        dependency.c (r1.54 -> r1.55)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/dependency.c.diff?r1=1.54&r2=1.55)
        heap.c (r1.299 -> r1.300)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/heap.c.diff?r1=1.299&r2=1.300)
    pgsql/src/backend/commands:
        tablecmds.c (r1.185 -> r1.186)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablecmds.c.diff?r1=1.185&r2=1.186)
    pgsql/src/include/catalog:
        dependency.h (r1.23 -> r1.24)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/dependency.h.diff?r1=1.23&r2=1.24)
        heap.h (r1.80 -> r1.81)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/heap.h.diff?r1=1.80&r2=1.81)