Convert "remote:" sysroots to "target:" and remove "remote:"

Convert "remote:" sysroots to "target:" and remove "remote:"

The functionality of "target:" sysroots is a superset of the
functionality of "remote:" sysroots.  This commit causes the
"set sysroot" command to rewrite "remote:" sysroots as "target:"
sysroots and replaces "remote:" specific code with "target:"
specific code where still necessary.

gdb/ChangeLog:

	* remote.h (REMOTE_SYSROOT_PREFIX): Remove definition.
	(remote_filename_p): Remove declaration.
	(remote_bfd_open): Likewise.
	* remote.c (remote_bfd_iovec_open): Remove function.
	(remote_bfd_iovec_close): Likewise.
	(remote_bfd_iovec_pread): Likewise.
	(remote_bfd_iovec_stat): Likewise.
	(remote_filename_p): Likewise.
	(remote_bfd_open): Likewise.
	* symfile.h (gdb_bfd_open_maybe_remote): Remove declaration.
	* symfile.c (separate_debug_file_exists): Use gdb_bfd_open.
	(gdb_bfd_open_maybe_remote): Remove function.
	(symfile_bfd_open):  Replace remote filename check with
	target filename check.
	(reread_symbols): Use gdb_bfd_open.
	* build-id.c (gdbcore.h): New include.
	(build_id_to_debug_bfd): Use gdb_bfd_open.
	* infcmd.c (attach_command_post_wait): Remove remote filename
	check.
	* solib.c (solib_find): Replace remote-specific handling with
	target-specific handling.  Update comments where necessary.
	(solib_bfd_open): Replace remote-specific handling with
	target-specific handling.
	(gdb_sysroot_changed): New function.
	(_initialize_solib): Call the above when gdb_sysroot changes.
	* windows-tdep.c (gdbcore.h): New include.
	(windows_xfer_shared_library): Use gdb_bfd_open.