pgsql: Split definitions for md5.c out of crypt.h and into their own

Log Message:
-----------
Split definitions for md5.c out of crypt.h and into their own header
libpq/md5.h, so that there's a clear separation between backend-only
definitions and shared frontend/backend definitions. (Turns out this
is reversing a bad decision from some years ago...) Fix up references
to crypt.h as needed. I looked into moving the code into src/port, but
the headers in src/include/libpq are sufficiently intertwined that it
seems more work than it's worth to do that.

Modified Files:
--------------
pgsql/src/backend/commands:
user.c (r1.171 -> r1.172)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/user.c.diff?r1=1.171&r2=1.172)
pgsql/src/backend/libpq:
crypt.c (r1.68 -> r1.69)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/crypt.c.diff?r1=1.68&r2=1.69)
hba.c (r1.151 -> r1.152)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/hba.c.diff?r1=1.151&r2=1.152)
ip.c (r1.35 -> r1.36)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/ip.c.diff?r1=1.35&r2=1.36)
md5.c (r1.32 -> r1.33)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/md5.c.diff?r1=1.32&r2=1.33)
pgsql/src/backend/utils/adt:
varlena.c (r1.147 -> r1.148)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/varlena.c.diff?r1=1.147&r2=1.148)
pgsql/src/include/libpq:
crypt.h (r1.34 -> r1.35)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/libpq/crypt.h.diff?r1=1.34&r2=1.35)
ip.h (r1.17 -> r1.18)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/libpq/ip.h.diff?r1=1.17&r2=1.18)
pgsql/src/interfaces/libpq:
fe-auth.c (r1.114 -> r1.115)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-auth.c.diff?r1=1.114&r2=1.115)

Added Files:
-----------
pgsql/src/include/libpq:
md5.h (r1.3)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/libpq/md5.h?rev=1.3&content-type=text/x-cvsweb-markup)