pgsql: Fix pq_getbyte_if_available() function.

Log Message:
-----------
Fix pq_getbyte_if_available() function. It was confused on what it
returns if no data is immediately available. Patch by me with numerous
fixes from Fujii Masao and Magnus Hagander.

Modified Files:
--------------
    pgsql/src/backend/libpq:
        be-secure.c (r1.96 -> r1.97)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/be-secure.c?r1=1.96&r2=1.97)
        pqcomm.c (r1.203 -> r1.204)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/pqcomm.c?r1=1.203&r2=1.204)
    pgsql/src/backend/replication:
        walsender.c (r1.6 -> r1.7)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/replication/walsender.c?r1=1.6&r2=1.7)