tree-wide: use LOG_MESSAGE() where possible

Also break some long lines for more uniform formatting. No functional change.

I went over all log_struct, log_struct_errno, log_unit_struct,
log_unit_struct_errno calls, and they seem fine.

src/basic/log.h

@@ -298,7 +298,8 @@ int log_emergency_level(void);
 
 bool log_on_console(void) _pure_;
 
-/* Helper to prepare various field for structured logging */
+/* Helper to wrap the main message in structured logging. The macro doesn't do much,
+ * except to provide visual grouping of the format string and its arguments. */
 #define LOG_MESSAGE(fmt, ...) "MESSAGE=" fmt, ##__VA_ARGS__
 
 void log_received_signal(int level, const struct signalfd_siginfo *si);

src/basic/user-util.c

@@ -794,7 +794,7 @@ bool valid_user_group_name(const char *u, ValidUserFlags flags) {
                 /* Compare with strict result and warn if result doesn't match */
                 if (FLAGS_SET(flags, VALID_USER_WARN) && !valid_user_group_name(u, 0))
                         log_struct(LOG_NOTICE,
-                                   "MESSAGE=Accepting user/group name '%s', which does not match strict user/group name rules.", u,
+                                   LOG_MESSAGE("Accepting user/group name '%s', which does not match strict user/group name rules.", u),
                                    "USER_GROUP_NAME=%s", u,
                                    "MESSAGE_ID=" SD_MESSAGE_UNSAFE_USER_NAME_STR);