Implement remote_bfd_iovec_stat

Implement remote_bfd_iovec_stat

This commit adds a new packet "vFile:fstat:" to the remote protocol
that can be used by to retrieve information about files that have
been previously opened using vFile:open.  vFile:fstat: support is
added to GDB, and remote_bfd_iovec_stat is implemented using it.  If
vFile:fstat: is not supported by the remote GDB creates a dummy result
by zeroing the supplied stat structure and setting its st_size field
to INT_MAX.  This mimics GDB's previous behaviour, with the exception
that GDB did not previously zero the structure so all other fields
would have been returned unchanged, which is to say very likely
populated with random values from the stack.

gdb/ChangeLog:

	* remote-fileio.h (remote_fileio_to_host_stat): New declaration.
	* remote-fileio.c (remote_fileio_to_host_uint): New function.
	(remote_fileio_to_host_ulong): Likewise.
	(remote_fileio_to_host_mode): Likewise.
	(remote_fileio_to_host_time): Likewise.
	(remote_fileio_to_host_stat): Likewise.
	* remote.c (PACKET_vFile_fstat): New enum value.
	(remote_protocol_features): Register the "vFile:fstat" feature.
	(remote_hostio_fstat): New function.
	(remote_bfd_iovec_stat): Use the above.
	(_initialize_remote): Register new "set/show remote
	hostio-fstat-packet" command.
	* symfile.c (separate_debug_file_exists): Update comment.
	* NEWS: Announce new vFile:fstat packet.

gdb/doc/ChangeLog:

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