cleanup: handle server FDs in a more generic way

This is still hackish, because the code relies on walking the FDs in
exactly the same order they were added, but moving in the right direction.
cleanup: split READMEM handling into send_literal

This approach is not very versatile, but any commands that need to send
a literal as the only data can benefit from it.
Add the COMMAND command

Provide a way to execute an arbitrary crash command.
clean up on module unload

This is usually not very useful, because all objects should already be
freed when the _fini routine is called, but let's keep it here as a
reminder if we ever make it possible to execute things like "extend -u"
via the protocol.
COMMAND: improve error handling

This setjmp looks messy, but I can't see any other solution to a failing
command...