Adds support for a configurable set of trusted directories to search
executables in (exec_dirs), which defaults to system PATH. If your
filter specifies an exec_path that doesn't start with '/', then it
will be searched in exec_dirs. Avoids having to write multiple
filters to care for distro differences. Fixes bug 1079723.
Also returns a specific error rather than try to run absent executables.
Change-Id: Idab03bb0be6832a75ffeed4e78d25d0543f5caf9
87fc023794