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_port_memory_int.h

@@ -100,7 +100,7 @@ nxt_inline void
 nxt_port_mmap_set_chunk_free(nxt_free_map_t *m, nxt_chunk_id_t c);
 
 nxt_inline nxt_chunk_id_t
-nxt_port_mmap_chunk_id(nxt_port_mmap_header_t *hdr, u_char *p)
+nxt_port_mmap_chunk_id(nxt_port_mmap_header_t *hdr, const u_char *p)
 {
     u_char  *mm_start;