Fix pg_isready to handle conninfo properly.

pg_isready displays the host name and the port number that it uses to connect
to the server. So far, pg_isready didn't use the conninfo specified in -d option
for calculating those host name and port number. This can lead to wrong display
to a user. This commit changes pg_isready so that it uses the conninfo for that
calculation.

Original patch by Phil Sorber, modified by me.
Add description that loread()/lowrite() are corresponding to
lo_read()/lo_write() in libpq to avoid confusion.