pgsql: Add a hook to CREATE/ALTER ROLE to allow an external module to

Log Message:
-----------
Add a hook to CREATE/ALTER ROLE to allow an external module to check the
strength of database passwords, and create a sample implementation of
such a hook as a new contrib module "passwordcheck".

Laurenz Albe, reviewed by Takahiro Itagaki

Modified Files:
--------------
pgsql/contrib:
Makefile (r1.89 -> r1.90)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/Makefile?r1=1.89&r2=1.90)
README (r1.101 -> r1.102)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/README?r1=1.101&r2=1.102)
pgsql/doc/src/sgml:
contrib.sgml (r1.14 -> r1.15)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/contrib.sgml?r1=1.14&r2=1.15)
filelist.sgml (r1.64 -> r1.65)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/filelist.sgml?r1=1.64&r2=1.65)
pgsql/src/backend/commands:
user.c (r1.189 -> r1.190)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/user.c?r1=1.189&r2=1.190)
pgsql/src/include/commands:
user.h (r1.30 -> r1.31)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/user.h?r1=1.30&r2=1.31)

Added Files:
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pgsql/contrib/passwordcheck:
Makefile (r1.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/passwordcheck/Makefile?rev=1.1&content-type=text/x-cvsweb-markup)
passwordcheck.c (r1.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/contrib/passwordcheck/passwordcheck.c?rev=1.1&content-type=text/x-cvsweb-markup)
pgsql/doc/src/sgml:
passwordcheck.sgml (r1.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/passwordcheck.sgml?rev=1.1&content-type=text/x-cvsweb-markup)