pgsql: Invent a new, more thread-safe version of PQrequestCancel, called

Log Message:
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Invent a new, more thread-safe version of PQrequestCancel, called PQcancel.
Use this new function in psql.  Implement query cancellation in psql for
Windows.  Code by Magnus Hagander, documentation and minor editorialization
by Tom Lane.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        libpq.sgml (r1.166 -> r1.167)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/libpq.sgml.diff?r1=1.166&r2=1.167)
    pgsql/src/bin/psql:
        common.c (r1.92 -> r1.93)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/common.c.diff?r1=1.92&r2=1.93)
        common.h (r1.39 -> r1.40)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/common.h.diff?r1=1.39&r2=1.40)
        mainloop.c (r1.64 -> r1.65)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/mainloop.c.diff?r1=1.64&r2=1.65)
    pgsql/src/interfaces/libpq:
        exports.txt (r1.2 -> r1.3)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/exports.txt.diff?r1=1.2&r2=1.3)
        fe-connect.c (r1.288 -> r1.289)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/fe-connect.c.diff?r1=1.288&r2=1.289)
        libpq-fe.h (r1.112 -> r1.113)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/libpq-fe.h.diff?r1=1.112&r2=1.113)
        libpq-int.h (r1.95 -> r1.96)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/libpq/libpq-int.h.diff?r1=1.95&r2=1.96)