Replace parse_datetime() constants by named flags

Remove the magic constants used in the return value of parse_datetime()
and use named flags instead.

Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>

src/calcurse.h

@@ -628,6 +628,10 @@ enum getstr {
 	GETSTRING_RET		/* return was pressed without entering any text. */
 };
 
+/* Return codes for parse_datetime(). */
+#define PARSE_DATETIME_HAS_DATE (1 << 0)
+#define PARSE_DATETIME_HAS_TIME (1 << 1)
+
 /* Week days. */
 enum wday {
 	SUNDAY,