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.

(trunk r8682)

svn path=/main/branches/2.1.2/; revision=8684