core: add generic "clean" operation to units

This adds basic infrastructure to implement a "clean" operation for unit
types. This "clean" operation is supposed to remove on-disk resources of
units, and is supposed to be used in a later commit to clean our
RuntimeDirectory=, StateDirectory= and so on of service units.

Later commits will open this up to the bus, and hook up service units
with this.

This also adds a new generic ActiveState called UNIT_MAINTENANCE. It's
supposed to cover all kinds of "maintainance" state of units.
Specifically, this is supposed to cover the "cleaning" operations later
added for service units which might take a bit of time. This high-level,
generic, abstract state is called UNIT_MAINTENANCE instead of the
more specific "UNIT_CLEANING", since I think this should be kept open
for different operations possibly later on that could be nicely subsumed
under this (for example, maybe a recursive chown()ing operation could be
covered by this, and similar).

src/basic/unit-def.c

@@ -102,7 +102,8 @@ static const char* const unit_active_state_table[_UNIT_ACTIVE_STATE_MAX] = {
         [UNIT_INACTIVE] = "inactive",
         [UNIT_FAILED] = "failed",
         [UNIT_ACTIVATING] = "activating",
-        [UNIT_DEACTIVATING] = "deactivating"
+        [UNIT_DEACTIVATING] = "deactivating",
+        [UNIT_MAINTENANCE] = "maintenance",
 };
 
 DEFINE_STRING_TABLE_LOOKUP(unit_active_state, UnitActiveState);

src/basic/unit-def.h

@@ -40,6 +40,7 @@ typedef enum UnitActiveState {
         UNIT_FAILED,
         UNIT_ACTIVATING,
         UNIT_DEACTIVATING,
+        UNIT_MAINTENANCE,
         _UNIT_ACTIVE_STATE_MAX,
         _UNIT_ACTIVE_STATE_INVALID = -1
 } UnitActiveState;