execute: Drop log level to unit log level in exec_spawn()

All messages logged from exec_spawn() are attributed to the unit
and as such we should set the log level to the unit's max log level
for the duration of the function.

src/basic/log.h

@@ -56,7 +56,7 @@ int log_set_target_from_string(const char *e);
 LogTarget log_get_target(void) _pure_;
 void log_settle_target(void);
 
-void log_set_max_level(int level);
+int log_set_max_level(int level);
 int log_set_max_level_from_string(const char *e);
 int log_get_max_level(void) _pure_;
 int log_max_levels_to_string(int level, char **ret);
@@ -491,6 +491,15 @@ size_t log_context_num_contexts(void);
 /* Returns the number of fields in all attached log contexts. */
 size_t log_context_num_fields(void);
 
+static inline void _reset_log_level(int *saved_log_level) {
+        assert(saved_log_level);
+
+        log_set_max_level(*saved_log_level);
+}
+
+#define LOG_CONTEXT_SET_LOG_LEVEL(level) \
+        _cleanup_(_reset_log_level) _unused_ int _saved_log_level_ = log_set_max_level(level);
+
 #define LOG_CONTEXT_PUSH(...) \
         LOG_CONTEXT_PUSH_STRV(STRV_MAKE(__VA_ARGS__))