shared/options: introduce OPTION_ERROR

[zjs: this was originally proposed in
https://github.com/systemd/systemd/commit/930fc9d6980f27b278527b0d6117f97296fcaf6a.
I'm rescuing one chunk from that patch.]

src/shared/options.h

@@ -80,6 +80,10 @@ typedef struct Option {
 #define OPTION_POSITIONAL OPTION_FULL(OPTION_POSITIONAL_ENTRY, /* sc= */ 0, "(positional)", /* mv= */ NULL, /* h= */ NULL)
 #define OPTION_HELP_VERBATIM(lc, h) OPTION_FULL(OPTION_HELP_ENTRY_VERBATIM, /* sc= */ 0, lc, /* mv= */ NULL, h)
 
+/* This can be used when custom error handling is needed. */
+#define OPTION_ERROR                                                    \
+        case INT_MIN ... -1
+
 #define OPTION_COMMON_HELP                                              \
         OPTION('h', "help", NULL, "Show this help")