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_string.c

@@ -338,7 +338,7 @@ nxt_rmemstrn(const u_char *s, const u_char *end, const char *ss, size_t length)
 
 
 size_t
-nxt_str_strip(u_char *start, u_char *end)
+nxt_str_strip(const u_char *start, u_char *end)
 {
     u_char  *p;