Bug #200313 - Detect and report when an ebuild phase

Bug #200313 - Detect and report when an ebuild phase
exits unexpectedly. This is type of behavior is known
to be triggered by things such as failed variable
assignments (bug #190128) or bad substitution errors
(bug #200313).

We use a EBUILD_EXIT_STATUS_FILE environment variable
to specify a file that the shell code is supposed to
create when it exits in a normal manner. If the file
does not get created like it's supposed to be then we
can conclude that the shell has exited in some
unexpected way.

svn path=/main/trunk/; revision=8682