pgsql: Add SET ROLE.

Log Message:
-----------
Add SET ROLE. This is a partial commit of Stephen Frost's recent patch;
I'm still working on the has_role function and information_schema changes.

Modified Files:
--------------
pgsql/doc/src/sgml:
func.sgml (r1.269 -> r1.270)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml.diff?r1=1.269&r2=1.270)
reference.sgml (r1.53 -> r1.54)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/reference.sgml.diff?r1=1.53&r2=1.54)
pgsql/doc/src/sgml/ref:
allfiles.sgml (r1.63 -> r1.64)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/allfiles.sgml.diff?r1=1.63&r2=1.64)
pg_dump.sgml (r1.79 -> r1.80)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/pg_dump.sgml.diff?r1=1.79&r2=1.80)
pg_dumpall.sgml (r1.51 -> r1.52)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/pg_dumpall.sgml.diff?r1=1.51&r2=1.52)
pg_restore.sgml (r1.53 -> r1.54)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/pg_restore.sgml.diff?r1=1.53&r2=1.54)
set_session_auth.sgml (r1.12 -> r1.13)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/set_session_auth.sgml.diff?r1=1.12&r2=1.13)
pgsql/src/backend/access/transam:
xact.c (r1.210 -> r1.211)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xact.c.diff?r1=1.210&r2=1.211)
pgsql/src/backend/commands:
user.c (r1.156 -> r1.157)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/user.c.diff?r1=1.156&r2=1.157)
variable.c (r1.111 -> r1.112)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/variable.c.diff?r1=1.111&r2=1.112)
pgsql/src/backend/parser:
gram.y (r2.501 -> r2.502)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y.diff?r1=2.501&r2=2.502)
pgsql/src/backend/utils/init:
miscinit.c (r1.146 -> r1.147)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/init/miscinit.c.diff?r1=1.146&r2=1.147)
pgsql/src/backend/utils/misc:
check_guc (r1.8 -> r1.9)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/check_guc.diff?r1=1.8&r2=1.9)
guc.c (r1.277 -> r1.278)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.277&r2=1.278)
pgsql/src/include/commands:
variable.h (r1.25 -> r1.26)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/variable.h.diff?r1=1.25&r2=1.26)
pgsql/src/include:
miscadmin.h (r1.177 -> r1.178)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/miscadmin.h.diff?r1=1.177&r2=1.178)

Added Files:
-----------
pgsql/doc/src/sgml/ref:
set_role.sgml (r1.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/set_role.sgml?rev=1.1&content-type=text/x-cvsweb-markup)