Allow deleted processes to be reaped by SIGCHLD handler on MS-Windows.

Allow deleted processes to be reaped by SIGCHLD handler on MS-Windows.

 src/w32proc.c (new_child): Free up to 2 slots of dead processes at a
 time.  Improve diagnostics in DebPrint.
 (reader_thread): If cp->char_avail is NULL, set the FILE_AT_EOF
 flag, so that sys_select could have a chance of noticing that this
 process is dead, and call a SIGCHLD handler for it.  Improve
 diagnostics in DebPrint.
 (reap_subprocess): Reset the FILE_AT_EOF flag set by
 reader_thread.
 (sys_select): Watch a process whose procinfo.hProcess is non-NULL
 even if its char_avail is NULL.  Allows to reap subprocesses that
 were forcibly deleted by delete-process.  (Bug#13546)