Implement remote_pid_to_exec_file using qXfer:exec-file:read

Implement remote_pid_to_exec_file using qXfer:exec-file:read

This commit adds a new packet "qXfer:exec-file:read" to the remote
protocol that can be used to obtain the pathname of the file that
was executed to create a process on the remote system.  Support for
this packet is added to GDB and remote_ops.to_pid_to_exec_file is
implemented using it.

gdb/ChangeLog:

	* target.h (TARGET_OBJECT_EXEC_FILE): New enum value.
	* remote.c (PACKET_qXfer_exec_file): Likewise.
	(remote_protocol_features): Register the
	"qXfer:exec-file:read" feature.
	(remote_xfer_partial): Handle TARGET_OBJECT_EXEC_FILE.
	(remote_pid_to_exec_file): New function.
	(init_remote_ops): Initialize to_pid_to_exec_file.
	(_initialize_remote): Register new "set/show remote
	pid-to-exec-file-packet" command.
	* NEWS: Announce new qXfer:exec-file:read packet.

gdb/doc/ChangeLog:

	* gdb.texinfo (Remote Configuration): Document the "set/show
	remote pid-to-exec-file-packet" command.
	(General Query Packets): Document the qXfer:exec-file:read
	qSupported features.  Document the qXfer:exec-file:read packet.