Abstract some daemon functionality

This patch abstracts the chdir/umask/setsid functionality, which is
identical in the code functions, just that Daemonize did the chdir/umask
in the second child; with this change it does it in the first, as
StartDaemon.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>