Fix pg_isready to handle -d option properly.

Previously, -d option for pg_isready was broken. When the name of the
database was specified by -d option, pg_isready failed with an error.
When the conninfo specified by -d option contained the setting of the
host name but not Numeric IP address (i.e., hostaddr), pg_isready
displayed wrong connection message. -d option could not handle a valid
URI prefix at all. This commit fixes these bugs of pg_isready.

Backpatch to 9.3, where pg_isready was introduced.

Per report from Josh Berkus and Robert Haas.
Original patch by Fabrízio de Royes Mello, heavily modified by me.