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.
    
    (cherry picked from commit 3572d3df8f822d4cf1601428401a837f723771cf)
    
    Related: #1830861
    
    patch_name: 0420-shared-add-generic-logic-for-waiting-for-a-unit-to-e.patch
    present_in_specfile: true
    location_in_specfile: 420
    squash_commits: true