provide and export Curl_parsedate() as a library-wide internal function

for a better API to date parsing than the external API is

lib/parsedate.h

@@ -26,4 +26,22 @@
 extern const char * const Curl_wkday[7];
 extern const char * const Curl_month[12];
 
+/*
+ * Curl_parsedate()
+ *
+ * Returns:
+ *
+ * PARSEDATE_OK     - a fine conversion
+ * PARSEDATE_FAIL   - failed to convert
+ * PARSEDATE_LATER  - time overflow at the far end of time_t
+ * PARSEDATE_SOONER - time underflow at the low end of time_t
+ */
+
+int Curl_parsedate(const char *date, time_t *output);
+
+#define PARSEDATE_OK     0
+#define PARSEDATE_FAIL   -1
+#define PARSEDATE_LATER  1
+#define PARSEDATE_SOONER 2
+
 #endif