Improve handling of crash dumps (issue #12236).

 * If the environment variable PHANTOMJS_DISABLE_CRASH_DUMPS
   is set to any value, do not initialize Breakpad.

 * On Mac and Linux, if the environment variable TMPDIR is set,
   put the crash dumps there instead of in /tmp.

 * On Windows, %TEMP% was already being honored, but streamline
   the code for that.

 * Move the code that initializes Breakpad, and the code to print
   the crash messages, to their own module (src/crashdump.{h,cpp})
   and RAII class (CrashHandler).

 * Better wording of the crash message, particularly in the case where
   Breakpad failed to write a minidump file; update URL of crash-reporting
   guide.

 * Generally less repetitive code.  (Still way too many #ifdefs.)
Convert Utils into a namespace and remove dead code.

Post-cleanup for #12236.