Wrap TEMP_FAILURE_RETRY around system calls

BUG 19934827

Wrap TEMP_FAILURE_RETRY around system calls that can return EINTR
(waitpid, close).

Refactor fork/exec flows in various places into a utility function
and log errors so we can better understand failures in the test server.

Fix a small use-after-free issue in ScriptGroups.

Change-Id: I60b192f83c395a13c27cd6bd2289c44132b84791
5 files changed