init: fix signal handling and LOG(FATAL) in child processes

Child processes inherit the signal handlers from their parent process.
In the case of init, fork()'ed processes, will attempt to reboot the
system if they receive a fatal signal).  This is not the correct behavior;
these processes should terminate due to the provided signal like other
processes on the system.

This is particularly important as there are multiple LOG(FATAL) calls
in service.cpp for failures after fork() but before execv() when a
service is started.

Note, that pthread_atfork() is not a viable solution since clone() is
used in some cases instead of fork() and atfork handlers are not
called with clone().

Bug: 65637054
Test: LOG(FATAL) from a child process of init and see that it
      terminates due to a signal correctly
Test: LOG(FATAL) from init proper and see that it reboots to the
      bootloader

Change-Id: I875ebd7a5f6b3f5e3e2c028af3306917c4409db3