Implement multiple-filesystem support for remote targets

Implement multiple-filesystem support for remote targets

This commit allows GDB to access executables and shared libraries
on remote targets where the remote stub does not share a common
filesystem with the inferior(s).  A new packet "vFile:setfs" is
added to the remote protocol and the three remote hostio functions
with filename arguments are modified to send "vFile:setfs" packets
as necessary.

gdb/ChangeLog:

	* remote.c (struct remote_state) <fs_pid>: New field.
	(new_remote_state): Initialize the above.
	(PACKET_vFile_setfs): New enum value.
	(remote_hostio_set_filesystem): New function.
	(remote_hostio_open): Call the above.
	(remote_hostio_unlink): Likewise.
	(remote_hostio_readlink): Likewise.
	(_initialize_remote): Register new "set/show remote
	hostio-setfs-packet" command.
	* NEWS: Announce new vFile:setfs packet.

gdb/doc/ChangeLog:

	* gdb.texinfo (Remote Configuration): Document the
	"set/show remote hostio-setfs-packet" command.
	(Host I/O Packets): Document the vFile:setfs packet.