exec: SystemCallLog= directive

With new directive SystemCallLog= it's possible to list system calls to be
logged. This can be used for auditing or temporarily when constructing system
call filters.

---
v5: drop intermediary, update HASHMAP_FOREACH_KEY() use
v4: skip useless debug messages, actually parse directive
v3: don't declare unused variables with old libseccomp
v2: fix build without seccomp or old libseccomp

src/systemctl/systemctl.c

@@ -4822,7 +4822,7 @@ static int print_property(const char *name, const char *expected_value, sd_bus_m
 
                         return 1;
 
-                } else if (STR_IN_SET(name, "SystemCallFilter", "RestrictAddressFamilies")) {
+                } else if (STR_IN_SET(name, "SystemCallFilter", "SystemCallLog", "RestrictAddressFamilies")) {
                         _cleanup_strv_free_ char **l = NULL;
                         int allow_list;