linux-nat: Exploit /proc/<pid>/mem for writing

linux-nat: Exploit /proc/<pid>/mem for writing

So far linux_proc_xfer_partial refused to handle write requests.  This is
still based on the assumption that the Linux kernel does not support
writes to /proc/<pid>/mem.  That used to be true, but has changed with
Linux 2.6.39 released in May 2011.

This patch lifts this restriction and now exploits /proc/<pid>/mem for
writing to inferior memory as well, if possible.

gdb/ChangeLog:

	* linux-nat.c (linux_proc_xfer_partial): Handle write operations
	as well.