Add a way to disable crash dumps (issue #12236).

 * If the environment variable PHANTOMJS_DISABLE_CRASH_DUMPS
   is set to any value, do not initialize Breakpad.  (On Windows,
   we still call _CrtSetReportMode(_CRT_ASSERT, 0).)

 * On Mac and Linux, if the environment variable TMPDIR is set,
   put the crash dumps there instead of in /tmp.

 * Move the code that initializes Breakpad to utils.cpp, so it is
   next to the code that prints the crash message.

Note: only tested on Linux, not even compiled anywhere else.  This change should be mentioned in the documentation somewhere, but I don't know where to put it.
Doh.  It's unfortunate that I have no way of testing on Windows.  Will fix.