Fix bug #12829 with aborts on MS-Windows when several child processes die.

Fix bug #12829 with aborts on MS-Windows when several child processes die.

 nt/inc/sys/wait.h: New file, with prototype of waitpid and
 definitions of macros it needs.
 nt/inc/ms-w32.h (wait): Don't define, 'wait' is not used anymore.
 (sys_wait): Remove prototype.
 nt/config.nt (HAVE_SYS_WAIT_H): Define to 1.

 src/w32proc.c (create_child): Don't clip the PID of the child
 process to fit into an Emacs integer, as this is no longer a
 restriction.
 (waitpid): Rename from sys_wait.  Emulate a Posix 'waitpid' by
 reaping only the process specified by PID argument, if that is
 positive.  Use PID instead of dead_child to know which process to
 reap.  Wait for the child to die only if WNOHANG is not in
 OPTIONS.
 (sys_select): Don't set dead_child.
 src/sysdep.c (wait_for_termination_1): Remove the WINDOWSNT portion,
 as it is no longer needed.
 src/process.c (waitpid, WUNTRACED) [!WNOHANG]: Remove definitions,
 no longer needed.
 (record_child_status_change): Remove the setting of
 record_at_most_one_child for the !WNOHANG case.