core: add systemd-executor binary

core: add systemd-executor binary

Currently we spawn services by forking a child process, doing a bunch
of work, and then exec'ing the service executable.

There are some advantages to this approach:

- quick: we immediately have access to all the enourmous amount of
  state simply by virtue of sharing the memory with the parent
- easy to refactor and add features
- part of the same binary, will never be out of sync

There are however significant drawbacks:

- doing work after fork and before exec is against glibc's supported
  case for several APIs we call
- copy-on-write trap: anytime any memory is touched in either parent
  or child, a copy of that page will be triggered
- memory footprint of the child process will be memory footprint of
  PID1, but using the cgroup memory limits of the unit

The last issue is especially problematic on resource constrained
systems where hard memory caps are enforced and swap is not allowed.
As soon as PID1 is under load, with no page out due to no swap, and a
service with a low MemoryMax= tries to start, hilarity ensues.

Add a new systemd-executor binary, that is able to receive all the
required state via memfd, deserialize it, prepare the appropriate
data structures and call exec_child.

Use posix_spawn which uses CLONE_VM + CLONE_VFORK, to ensure there is
no copy-on-write (same address space will be used, and parent process
will be frozen, until exec).
The sd-executor binary is pinned by FD on startup, so that we can
guarantee there will be no incompatibilities during upgrades.