This fixes two issues:

- values for mutiple header fields with the same name
  passed as arrays were converted to string;

- the type of field value wasn't preserved as required
  by specification.

Node.js processes didn't exit after the changes in b9f7635e6be2,
as the quit command from port wasn't handled by the module.

We increase the number to the Unit object so that it lives forever.
This is necessary so that the garbage collector does not delete the Unit object.

The nxt_murmur_hash2() generated 4-byte hash that was stored in uintptr_t,
which was 8 bytes long on 64-bit systems.  At each iteration, it took the
previous key and hashed it again.

The problem was that it took only the first 4 bytes of the key, and these
4 bytes were always zero on 64-bit big-endian system.  That resulted in
equal keys at each iteration.

The bug was discovered on IBM/S390x.

The nxt_conf_map_object() function used nxt_int_t for NXT_CONF_MAP_INT, which
was 8 bytes long on 64-bit systems.

But the nxt_port_main_start_worker_handler() used it to map into the int field
of the nxt_common_app_conf_t structure, which was 4 bytes.  As the result, on
a 64-bit big-endian system all the meaningful module type numbers were assigned
into the gap above the "type" field.

The bug was discovered on IBM/S390x.

This closes #182 issue on GitHub.
Thanks to 洪志道 (Hong Zhi Dao).

The issue was caused by misplacement of allocated blocks in rbtree due
to broken comparison function if the distance between two allocations
did not fit into intptr_t.  As the result, nxt_mp_free() could have
failed to find the allocation.

In particular, it was mostly observed when Unit was compiled with
musl C library on 32-bits systems.

This closes #118 issue on GitHub.