Add battery support to all Cygwin builds

Add battery support to all Cygwin builds

It already exists in the Cygwin-w32 build.
* src/w32fns.c (Fw32_battery_status): Move to...
* src/w32cygwinx: New file, to be used for functions common to
the MS Windows and Cygwin builds.
(syms_of_w32cygwinx): New function.
* src/lisp.h: Add prototype of syms_of_w32cygwinx.
* src/emacs.c (main) [HAVE_NTGUI or CYGWIN]: Call syms_of_w32cygwinx.
* src/Makefile.in (SOME_MACHINE_OBJECTS):
* configure.ac (W32_OBJ) [HAVE_W32]: Add w32cygwinx.o.
(W32_LIBS) [CYGWIN]: Set equal to "-lkernel32" in non-w32 build.
(W32_OBJ) [CYGWIN]: Set equal to "w32cygwinx.o" in non-w32 build.
* etc/NEWS: Document the new battery support.