Extend "set cwd" to work on gdbserver

This is the "natural" extension necessary for the "set cwd" command
(and the whole "set the inferior's cwd" logic) to work on gdbserver.

The idea here is to have a new remote packet, QSetWorkingDir (name
adopted from LLDB's extension to the RSP, as can be seen at
<https://raw.githubusercontent.com/llvm-mirror/lldb/master/docs/lldb-gdb-remote.txt>),
which sends an hex-encoded string representing the working directory
that the remote inferior will use.  There is a slight difference from
the packet proposed by LLDB: GDB's version will accept empty
arguments, meaning that the user wants to clear the previously set
working directory for the inferior (i.e., "set cwd" without arguments
on GDB).

For UNIX-like targets this feature is already implemented on
nat/fork-inferior.c, and all gdbserver has to do is to basically
implement "set_inferior_cwd" and call it whenever such packet arrives.
For other targets, like Windows, it is possible to use the existing
"get_inferior_cwd" function and do the necessary steps to make sure
that the inferior will use the specified working directory.

Aside from that, the patch consists basically of updates to the
testcase (making it available on remote targets) and the
documentation.

No regressions found.

gdb/ChangeLog:
2017-10-04  Sergio Durigan Junior  <sergiodj@redhat.com>

	* NEWS (Changes since GDB 8.0): Add entry about new
	'set-cwd-on-gdbserver' feature.
	(New remote packets): Add entry for QSetWorkingDir.
	* common/common-inferior.h (set_inferior_cwd): New prototype.
	* infcmd.c (set_inferior_cwd): Remove "static".
	(show_cwd_command): Expand text to include remote debugging.
	* remote.c: Add PACKET_QSetWorkingDir.
	(remote_protocol_features) <QSetWorkingDir>: New entry for
	PACKET_QSetWorkingDir.
	(extended_remote_set_inferior_cwd): New function.
	(extended_remote_create_inferior): Call
	"extended_remote_set_inferior_cwd".
	(_initialize_remote): Call "add_packet_config_cmd" for
	QSetWorkingDir.

gdb/gdbserver/ChangeLog:
2017-10-04  Sergio Durigan Junior  <sergiodj@redhat.com>

	* inferiors.c (set_inferior_cwd): New function.
	* server.c (handle_general_set): Handle QSetWorkingDir packet.
	(handle_query): Inform that QSetWorkingDir is supported.
	* win32-low.c (create_process): Pass the inferior's cwd to
	CreateProcess.

gdb/testsuite/ChangeLog:
2017-10-04  Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdb.base/set-cwd.exp: Make it available on
	native-extended-gdbserver.

gdb/doc/ChangeLog:
2017-10-04  Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdb.texinfo (Starting your Program) <The working directory.>:
	Mention remote debugging.
	(Working Directory) <Your Program's Working Directory>:
	Likewise.
	(Connecting) <Remote Packet>: Add "set-working-dir"
	and "QSetWorkingDir" to the table.
	(Remote Protocol) <QSetWorkingDir>: New item, explaining the
	packet.

gdb/infcmd.c

@@ -242,10 +242,9 @@ show_args_command (struct ui_file *file, int from_tty,
   deprecated_show_value_hack (file, from_tty, c, get_inferior_args ());
 }
 
-/* Set the inferior current working directory.  If CWD is NULL, unset
-   the directory.  */
+/* See common/common-inferior.h.  */
 
-static void
+void
 set_inferior_cwd (const char *cwd)
 {
   struct inferior *inf = current_inferior ();
@@ -289,7 +288,8 @@ show_cwd_command (struct ui_file *file, int from_tty,
     fprintf_filtered (gdb_stdout,
 		      _("\
 You have not set the inferior's current working directory.\n\
-The inferior will inherit GDB's cwd.\n"));
+The inferior will inherit GDB's cwd if native debugging, or the remote\n\
+server's cwd if remote debugging.\n"));
   else
     fprintf_filtered (gdb_stdout,
 		      _("Current working directory that will be used "