init: cleanup exit() uses

Primarily, this fixes a bug where a forked child of property service
uses exit() instead of _exit, which has the unintended consequences of
running the global destructors of init proper, which leads to
unintended cleanup.

Secondly, this replaces the remaining calls of exit() that really
should be LOG(FATAL).

Test: boot sailfish
Change-Id: I779228e7d44a73186bc7685bb723c4b9278a0a2d