shared: add generic logic for waiting for a unit to enter some state

shared: add generic logic for waiting for a unit to enter some state

This is a generic implementation of a client-side logic of waiting until
a unit enters or leaves some state.

This is a more generic implementation of the WaitContext logic currently
in systemctl.c, and is supposed to replace it (a later commit does
this). It's similar to bus-wait-for-jobs.c and we probably should fold
that one into it later on.

This code is more powerful and cleaner than the WaitContext logic
however. In addition to waiting for a unit to exit this also allows us
to wait for a unit to leave the "maintainance" state.

This commit only implements the generic logic, and adds no users of it
yet.