pgsql: Disallow changing DEFAULT expression of a SERIAL column.

Log Message:
-----------
Disallow changing DEFAULT expression of a SERIAL column.

Dhanaraj M

Modified Files:
--------------
    pgsql/src/backend/catalog:
        dependency.c (r1.51 -> r1.52)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/dependency.c.diff?r1=1.51&r2=1.52)
        heap.c (r1.296 -> r1.297)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/heap.c.diff?r1=1.296&r2=1.297)
    pgsql/src/backend/commands:
        tablecmds.c (r1.181 -> r1.182)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablecmds.c.diff?r1=1.181&r2=1.182)
    pgsql/src/include/catalog:
        dependency.h (r1.21 -> r1.22)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/dependency.h.diff?r1=1.21&r2=1.22)
        heap.h (r1.78 -> r1.79)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/heap.h.diff?r1=1.78&r2=1.79)