Fix parsing of integer values for connection parameters in libpq

Commit e7a22179 has introduced stricter checks for integer values in
connection parameters for libpq.  However this failed to correctly check
after trailing whitespaces, while leading whitespaces were discarded per
the use of strtol(3).  This fixes and refactors the parsing logic to
handle both cases consistently.  Note that trying to restrict the use of
trailing whitespaces can easily break connection strings like in ECPG
regression tests (these have allowed me to catch the parsing bug with
connect_timeout).

Author: Michael Paquier
Reviewed-by: Lars Kanis
Discussion: https://postgr.es/m/a9b4cbd7-4ecb-06b2-ebd7-1739bbff3217@greiz-reinsdorf.de
Backpatch-through: 12