Constified numerous function parameters.

As was pointed out by the cppcheck[0] static code analysis utility we
can mark numerous function parameters as 'const'. This acts as a hint to
the compiler about our intentions and the compiler will tell us when we
deviate from them.

[0]: https://cppcheck.sourceforge.io/

src/nxt_http_parse.h

@@ -35,7 +35,7 @@ typedef union {
 
 struct nxt_http_request_parse_s {
     nxt_int_t                 (*handler)(nxt_http_request_parse_t *rp,
-                                         u_char **pos, u_char *end);
+                                         u_char **pos, const u_char *end);
 
     nxt_str_t                 method;