pgsql: Improve performance of CHECK_FOR_INTERRUPTS() macro on Windows by

Log Message:
-----------
Improve performance of CHECK_FOR_INTERRUPTS() macro on Windows by not doing
a kernel call unless there's some evidence of a pending signal.  This should
bring its performance on Windows into line with the Unix version.  Problem
diagnosis and patch by Qingqing Zhou.  Minor stylistic tweaks by moi ...
if it's broken, it's my fault.

Modified Files:
--------------
    pgsql/src/backend/port/win32:
        signal.c (r1.12 -> r1.13)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/port/win32/signal.c.diff?r1=1.12&r2=1.13)
    pgsql/src/include:
        miscadmin.h (r1.180 -> r1.181)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/miscadmin.h.diff?r1=1.180&r2=1.181)
    pgsql/src/include/port:
        win32.h (r1.47 -> r1.48)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/port/win32.h.diff?r1=1.47&r2=1.48)