Fix leak of regex string representations

The lexer returns a copy of a regular expression's string
representation which is then leaked during parsing. Since the only
consumer of that copy makes a copy of the copy (makedfa via mkdfa),
just return a pointer to the original string.

Test with:

    valgrind --leak-check=full a.out '/123/; /456/' </dev/null

Reported by Todd C. Miller.

lex.c

@@ -545,7 +545,7 @@ int regexpr(void)
 	*bp = 0;
 	if (c == 0)
 		SYNTAX("non-terminated regular expression %.10s...", buf);
-	yylval.s = tostring(buf);
+	yylval.s = buf;
 	unput('/');
 	RET(REGEXPR);
 }