…on SIGTERM"

* src/mono/mono/mini/mini-posix.c
  - Add signal handler for SIGTERM

* src/mono/mono/mini/mini-windows.c
  - Add signal handler for SIGTERM

* src/mono/mono/mini/mini-runtime.c
  - Add mono_sigterm_signal_handler to process SIGTERM that will set a global variable
    to be monitored by the GC finalizer thread

* src/mono/mono/mini/mini-runtime.h
  - Define prototype for mono_sigterm_signal_handler()

* src/mono/mono/metadata/gc.c
  - Monitor for sigterm and kick off the shutdown process when encountered by calling mono_runtime_try_shutdown().
  - Exit with either the user set exitcode (System.Environment.ExitCode) or SIGTERM + 128.

  - Re-enable ExitCodeTests for mono

  - Set a default exit code before starting the SIGTERM processing

  - Set a default exit code before setting the term_signaled variable that gets checked in gc

  - Simplify use of exit code now that a default is being set

  - Rename term_signaled to match mono style
  - Remove volatile attribute
  - Move testing of shutdown until after the sem wait

* src/mono/mono/mini/mini-runtime.c
  - Rename term_signaled to mono_term_signaled

* src/mono/mono/mini/mini-windows.c
  - Use the correct signal for handler