Fix adb hang when subprocess dies early.

* Handling of the subprocess and its FD.
  This fixes http://b/3400254

 "Many bugreports getting hung at the end in monkey"
  -  Start up a service thread that waits on the subprocess to terminate,
then closes the FD associated with it.
  -  Have the event handler select() with a timeout so that it can
  detect the closed FD. Select() with no timeout does not return when an FD is closed.
  - Have the event handler force a read on the closed FD to trigger the close sequence.
  - Migrate the "shell:blabla" handling to "#if !ADB_HOST" sections.
* Fix the race around OOM adjusting.
  - Do it in the child before exec() instead of the in the parent as the
   child could already have started or not (no /proc/pid/... yet).
* Allow for multi-threaded D() invocations to not clobber each other.
  - Allow locks across object files.
  - Add lock within D()
* Add some missing close(fd) calls
  - Match similar existing practices near dup2()
* Add extra D() invocations related to FD handling.
* Warn about using debugging as stderr/stdout is used for protocol.

Change-Id: Ie5c4a5e6bfbe3f22201adf5f9a205d32e069bf9d
Signed-off-by: JP Abgrall <jpa@google.com>