tree-wide: use -EINVAL for enum invalid values

As suggested in https://github.com/systemd/systemd/pull/11484#issuecomment-775288617.

This does not touch anything exposed in src/systemd. Changing the defines there
would be a compatibility break.

Note that tests are broken after this commit. They will be fixed in the next one.

src/basic/log.h

@@ -23,7 +23,7 @@ typedef enum LogTarget{
         LOG_TARGET_AUTO, /* console if stderr is not journal, JOURNAL_OR_KMSG otherwise */
         LOG_TARGET_NULL,
         _LOG_TARGET_MAX,
-        _LOG_TARGET_INVALID = -1
+        _LOG_TARGET_INVALID = -EINVAL,
 } LogTarget;
 
 /* Note to readers: << and >> have lower precedence than & and | */