[GDBserver] hostio.c: Fallback to packet buffer size if PATH_MAX is not available.

[GDBserver] hostio.c: Fallback to packet buffer size if PATH_MAX is not available.

PATH_MAX is not defined on systems which have no limit on filename
length, such as GNU/Hurd.  As designed, the hostio RSP packets carry
the paths as parameters in the request/reply packets, which themselves
have an upper size limit, so lifting the filename limit completely
would require a redesign with new hostio packets.  While that doesn't
happen, we can at least support filename lengths as long as the packet
buffer can fit.

gdb/gdbserver/
2013-07-01  Pedro Alves  <palves@redhat.com>

	* hostio.c (HOSTIO_PATH_MAX): Define.
	(require_filename, handle_open, handle_unlink, handle_readlink):
	Use it.