Make gdb_bfd_open able to open BFDs using target fileio

Make gdb_bfd_open able to open BFDs using target fileio

This commit updates gdb_bfd_open to access files using target
fileio functions if the supplied path starts with "target:"
and if the local and target filesystems are not the same.
This allows users to specify "set sysroot target:" and have
GDB access files locally or from the remote as appropriate.

The new functions in gdb_bfd.c are copies of functions from
remote.c. This duplication is intentional and will be removed
by the next commit in this series.

gdb/ChangeLog:

	* gdb/gdb_bfd.h (TARGET_SYSROOT_PREFIX): New definition.
	(is_target_filename): New declaration.
	(gdb_bfd_has_target_filename): Likewise.
	(gdb_bfd_open): Update documentation comment.
	* gdb_bfd.c (target.h): New include.
	(gdb/fileio.h): Likewise.
	(is_target_filename): New function.
	(gdb_bfd_has_target_filename): Likewise.
	(fileio_errno_to_host): Likewise.
	(gdb_bfd_iovec_fileio_open): Likewise.
	(gdb_bfd_iovec_fileio_pread): Likewise.
	(gdb_bfd_iovec_fileio_close): Likewise.
	(gdb_bfd_iovec_fileio_fstat): Likewise.
	(gdb_bfd_open): Use target fileio to access paths prefixed
	with "target:" where necessary.