psql: fix \connect with URIs and conninfo strings

This is the second try at this, after fcef161729 failed miserably and
had to be reverted: as it turns out, libpq cannot depend on libpgcommon
after all. Instead of shuffling code in the master branch, make that one
just like 9.4 and accept the duplication.  (This was all my own mistake,
not the patch submitter's).

psql was already accepting conninfo strings as the first parameter in
\connect, but the way it worked wasn't sane; some of the other
parameters would get the previous connection's values, causing it to
connect to a completely unexpected server or, more likely, not finding
any server at all because of completely wrong combinations of
parameters.

Fix by explicitely checking for a conninfo-looking parameter in the
dbname position; if one is found, use its complete specification rather
than mix with the other arguments.  Also, change tab-completion to not
try to complete conninfo/URI-looking "dbnames" and document that
conninfos are accepted as first argument.

There was a weak consensus to backpatch this, because while the behavior
of using the dbname as a conninfo is nowhere documented for \connect, it
is reasonable to expect that it works because it does work in many other
contexts.  Therefore this is backpatched all the way back to 9.0.

Author: David Fetter, Andrew Dunstan.  Some editorialization by me
(probably earning a Gierth's "Sloppy" badge in the process.)
Reviewers: Andrew Gierth, Erik Rijkers, Pavel Stěhule, Stephen Frost,
Robert Haas, Andrew Dunstan.

src/interfaces/libpq/fe-connect.c

@@ -4197,6 +4197,8 @@ parse_connection_string(const char *connstr, PQExpBuffer errorMessage,
  * designators.
  *
  * Returns the URI prefix length, 0 if the string doesn't contain a URI prefix.
+ *
+ * XXX this is duplicated in psql/common.c.
  */
 static int
 uri_prefix_length(const char *connstr)
@@ -4218,6 +4220,8 @@ uri_prefix_length(const char *connstr)
  *
  * Must be consistent with parse_connection_string: anything for which this
  * returns true should at least look like it's parseable by that routine.
+ *
+ * XXX this is duplicated in psql/common.c
  */
 static bool
 recognized_connection_string(const char *connstr)