core: rework how transient unit files and property drop-ins work

With this change the logic for placing transient unit files and drop-ins
generated via "systemctl set-property" is reworked.

The latter are now placed in the newly introduced "control" unit file
directory. The fomer are now placed in the "transient" unit file directory.

Note that the properties originally set when a transient unit was created will
be written to and stay in the transient unit file directory, while later
changes are done via drop-ins.

This is preparation for a later "systemctl revert" addition, where existing
drop-ins are flushed out, but the original transient definition is restored.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/basic/strv.h b/src/basic/strv.h
index 7bfa54408d..f61bbb5386 100644
--- a/src/basic/strv.h
+++ b/src/basic/strv.h
@@ -50,6 +50,7 @@ int strv_extend_strv(char ***a, char **b, bool filter_duplicates);
 int strv_extend_strv_concat(char ***a, char **b, const char *suffix);
 int strv_extend(char ***l, const char *value);
 int strv_extendf(char ***l, const char *format, ...) _printf_(2,0);
+int strv_extend_front(char ***l, const char *value);
 int strv_push(char ***l, char *value);
 int strv_push_pair(char ***l, char *a, char *b);
 int strv_push_prepend(char ***l, char *value);