core: clean-up signal reset logic when reexec

There's no need to save the old sigmask, if we are going to die. Let's
simplify this. Also, reset all the signal handlers, so that we don't
leave SIG_IGN set for some of them across reexec.
sd-dhcp-server: remove unused cleanup function

Removes _cleanup_dhcp_lease_free_. While the automatic cleanup
functions are great to have this one is never used and causes
a warning in clang.