pgsql: Create typedef pgsocket for storing socket descriptors.

Log Message:
-----------
Create typedef pgsocket for storing socket descriptors.

This silences some warnings on Win64. Not using the proper SOCKET datatype
was actually wrong on Win32 as well, but didn't cause any warnings there.

Also create define PGINVALID_SOCKET to indicate an invalid/non-existing
socket, instead of using a hardcoded -1 value.

Modified Files:
--------------
pgsql/src/backend/libpq:
auth.c (r1.190 -> r1.191)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/auth.c?r1=1.190&r2=1.191)
ip.c (r1.49 -> r1.50)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/ip.c?r1=1.49&r2=1.50)
pqcomm.c (r1.200 -> r1.201)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/pqcomm.c?r1=1.200&r2=1.201)
pgsql/src/backend/postmaster:
pgstat.c (r1.196 -> r1.197)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgstat.c?r1=1.196&r2=1.197)
postmaster.c (r1.599 -> r1.600)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/postmaster.c?r1=1.599&r2=1.600)
pgsql/src/include:
port.h (r1.128 -> r1.129)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/port.h?r1=1.128&r2=1.129)
pgsql/src/include/libpq:
libpq-be.h (r1.72 -> r1.73)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/libpq/libpq-be.h?r1=1.72&r2=1.73)
libpq.h (r1.72 -> r1.73)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/libpq/libpq.h?r1=1.72&r2=1.73)
pgsql/src/port:
noblock.c (r1.14 -> r1.15)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/port/noblock.c?r1=1.14&r2=1.15)