Cygwin: posix timers: reimplement using OS timer

- Rename files timer.* to posix_timer.*.
- Reimplement using an OS timer rather than a handcrafted wait loop.
- Use a Slim R/W Lock for synchronization.
- Drop timer chaining.  It doesn't server a purpose since all timers
  are local only.
- Rename ttstart to itimer_tracker to better reflect its purpose.
  It's not the anchor for a timer chain anymore anyway.
- Drop fixup_timers_after_fork.  Everything is process-local, nothing
  gets inherited.
- Rename timer_tracker::disarm_event to disarm_overrun_event for
  better readability.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/fork.cc

@@ -22,7 +22,6 @@ details. */
 #include "tls_pbuf.h"
 #include "dll_init.h"
 #include "cygmalloc.h"
-#include "timer.h"
 #include "ntdll.h"
 
 #define NPIDS_HELD 4
@@ -196,7 +195,6 @@ frok::child (volatile char * volatile here)
   ForceCloseHandle1 (fork_info->forker_finished, forker_finished);
 
   pthread::atforkchild ();
-  fixup_timers_after_fork ();
   cygbench ("fork-child");
   ld_preload ();
   fixup_hooks_after_fork ();