gdbserver:server.c - use unpack_varlen_hex to extract hex numbers.

Addresses, as most numbers in the RSP are hex encoded, with variable
length (that just means the width isn't specified, and there's no top
cap.  So they should be extracted with unpack_varlen_hex.

A couple spots in server.c are using strto(u)l, which doesn't work on
LLP64 targets.

This patch fixes it.

Tested on x86_64 Fedora 17.

2013-02-19  Pedro Alves  <palves@redhat.com>
	    Kai Tietz <ktietz@redhat.com>

	PR gdb/15161

	* server.c (handle_query) <CRC check>: Use unpack_varlen_hex
	instead of strtoul to extract address from packet.
	(process_serial_event) <'z'>: Likewise.