install: introduce a new unit file state "transient"

Now, that the search path logic knows the unit path for transient units we also
can introduce an explicit unit file state "transient" that clarifies to the
user what kind of unit file he is encountering.

src/systemctl/systemctl.c

@@ -5444,7 +5444,7 @@ static int enable_unit(int argc, char *argv[], void *userdata) {
                 if (r == -ESHUTDOWN)
                         return log_error_errno(r, "Unit file is masked.");
                 if (r == -EADDRNOTAVAIL)
-                        return log_error_errno(r, "Unit file is generated.");
+                        return log_error_errno(r, "Unit file is transient or generated.");
                 if (r < 0)
                         return log_error_errno(r, "Operation failed: %m");
 
@@ -5612,7 +5612,7 @@ static int add_dependency(int argc, char *argv[], void *userdata) {
                 if (r == -ESHUTDOWN)
                         return log_error_errno(r, "Unit file is masked.");
                 if (r == -EADDRNOTAVAIL)
-                        return log_error_errno(r, "Unit file is generated.");
+                        return log_error_errno(r, "Unit file is transient or generated.");
                 if (r < 0)
                         return log_error_errno(r, "Can't add dependency: %m");