log: add an "error" parameter to all low-level logging calls and intrdouce log_error_errno() as log calls that take error numbers

This change has two benefits:

- The format string %m will now resolve to the specified error (or to
  errno if the specified error is 0. This allows getting rid of a ton of
  strerror() invocations, a function that is not thread-safe.

- The specified error can be passed to the journal in the ERRNO= field.

Now of course, we just need somebody to convert all cases of this:

        log_error("Something happened: %s", strerror(-r));

into thus:

        log_error_errno(-r, "Something happened: %m");

src/core/kmod-setup.c

@@ -44,7 +44,7 @@ static void systemd_kmod_log(
 
         /* library logging is enabled at debug only */
         DISABLE_WARNING_FORMAT_NONLITERAL;
-        log_metav(LOG_DEBUG, file, line, fn, format, args);
+        log_metav(LOG_DEBUG, 0, file, line, fn, format, args);
         REENABLE_WARNING;
 }
 

src/core/selinux-access.c

@@ -112,7 +112,7 @@ _printf_(2, 3) static int log_callback(int type, const char *fmt, ...) {
 #endif
 
         va_start(ap, fmt);
-        log_metav(LOG_USER | LOG_INFO, __FILE__, __LINE__, __FUNCTION__, fmt, ap);
+        log_metav(LOG_USER | LOG_INFO, 0, __FILE__, __LINE__, __FUNCTION__, fmt, ap);
         va_end(ap);
 
         return 0;

src/core/unit.h

@@ -596,7 +596,9 @@ UnitActiveState unit_active_state_from_string(const char *s) _pure_;
 
 /* Macros which append UNIT= or USER_UNIT= to the message */
 
-#define log_full_unit(level, unit, ...) log_meta_object(level, __FILE__, __LINE__, __func__, getpid() == 1 ? "UNIT=" : "USER_UNIT=", unit, __VA_ARGS__)
+#define log_full_unit(level, unit, ...) log_meta_object(level, 0, __FILE__, __LINE__, __func__, getpid() == 1 ? "UNIT=" : "USER_UNIT=", unit, __VA_ARGS__)
+#define log_full_unit_errno(level, error, unit, ...) log_meta_object(level, error, __FILE__, __LINE__, __func__, getpid() == 1 ? "UNIT=" : "USER_UNIT=", unit, __VA_ARGS__)
+
 #define log_debug_unit(unit, ...)       log_full_unit(LOG_DEBUG, unit, __VA_ARGS__)
 #define log_info_unit(unit, ...)        log_full_unit(LOG_INFO, unit, __VA_ARGS__)
 #define log_notice_unit(unit, ...)      log_full_unit(LOG_NOTICE, unit, __VA_ARGS__)