Support SIGTRAP in kill emulation on Windows

Support SIGTRAP in kill emulation on Windows

* src/w32proc.c (sys_kill): Translate SIGTRAP signal into a call to
'DebugBreakProcess' to cause a breakpoint exception to occur in the
specified process.  On Windows versions prior to Windows XP that do
not support 'DebugBreakProcess' return -1 and set errno to ENOTSUP
(as opposed to EINVAL before this change).
* src/w32proc.c: Add typedef for 'DebugBreakProcess' function pointer
and global variable to track state of run-time dynamic linking of this
function.

* etc/NEWS: Add entry to document that 'signal-process' now supports
SIGTRAP.