Process::Start() will fork() and execvp() the child process. For use-cases
where the child process must have specific signal masks or other properties
before the execvp() call this is unfeasable, the caller cannot control the
properties of the child between forking and execvp.

Split the fork() call out into Process::Fork(), making it optional. Start()
will fork on demand if it hasn't been called before. Behaviour stays the
same for callers of Start(), only those that call Fork() first need to pay
attention to details.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Chase Douglas <chase.douglas@ubuntu.com>