added qse_xli_definepair() and qse_xli_undefinepair().

renamed some xli functions.
updated cmd/httpd to utilize qse_xli_definepair()

qse/lib/xli/err.c

@@ -49,8 +49,10 @@ const qse_char_t* qse_xli_dflerrstr (
 		QSE_T("invalid character '${0}'"),
 		QSE_T("'${0}' not recognized"),
 		QSE_T("@ not followed by a valid word"),
-		QSE_T("illegal key '${0}'"),
+		QSE_T("undefined key '${0}'"),
+		QSE_T("no alias for '${0}'"),
 		QSE_T("illegal value for '${0}'"),
+		QSE_T("no value for '${0}'"),
 		QSE_T("too many string segments for '${0}'")
 	};