Reimplement "vFile:fstat" without qSupported

Reimplement "vFile:fstat" without qSupported

This commit makes support for the "vFile:fstat" packet be detected
by probing rather than using qSupported, for consistency with the
other vFile: packets.

gdb/ChangeLog:

	(remote_protocol_features): Remove the "vFile:fstat" feature.
	(remote_hostio_fstat): Probe for "vFile:fstat" support.

gdb/doc/ChangeLog:

	* gdb.texinfo (General Query Packets): Remove documentation
	for now-removed vFile:fstat qSupported features.

gdb/gdbserver/ChangeLog:

	* server.c (handle_query): Do not report vFile:fstat as supported.