[patch] Suggest newer gdbserver if it has no qXfer:exec-file:read

[Jan Kratochvil <jan.kratochvil@redhat.com>]

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Hi,

currently:
	$ gdbserver-7.9 :1234 true &
	$ gdb -q -ex 'target remote :1234' # that -q is not relevant here
	Remote debugging using :1234
	warning: Could not load vsyscall page because no executable was specified
	try using the "file" command first.
	0x00007ffff7ddcc80 in ?? ()
	(gdb) b main
	No symbol table is loaded.  Use the "file" command.
	Make breakpoint pending on future shared library load? (y or [n]) _

While one may not realize a newer gdbserver would fix that:
	$ gdbserver-7.10 :1234 true &
	$ gdb -q -ex 'target remote :1234' # that -q is not relevant here
	Remote debugging using :1234
	Reading /usr/bin/true from remote target...
	warning: File transfers from remote targets can be slow. Use "set sysroot" to access files locally instead.
	Reading /usr/bin/true from remote target...
	Reading symbols from target:/usr/bin/true...Reading symbols from /usr/lib/debug/usr/bin/true.debug...done.
	done.
	Reading /lib64/ld-linux-x86-64.so.2 from remote target...
	Reading /lib64/ld-linux-x86-64.so.2 from remote target...
	Reading symbols from target:/lib64/ld-linux-x86-64.so.2...Reading symbols from /usr/lib/debug/usr/lib64/ld-2.22.so.debug...done.
	done.
	0x00007ffff7ddcc80 in _start () from target:/lib64/ld-linux-x86-64.so.2
	(gdb) b main
	Breakpoint 1 at 0x555555555650: file src/true.c, line 59.
	(gdb) _

This can be more common case with the popular containers.  Therefore
suggesting to print there also:
	warning: No executable has been specified (see the "file" command) and remote gdbserver does not support packet "qXfer:exec-file:read" - please use FSF gdbserver version 7.10 or later.

OK for check-in?

No regressions on {x86_64,x86_64-m32,i686}-fedora23-linux-gnu.

The "qXfer:exec-file:read" support in GDB and gdbserver was implemented by:
	commit c78fa86a213db1bdef328437ac262a4f54577827
	Author: Gary Benson <gbenson@redhat.com>
	Date:   Fri Apr 17 09:47:30 2015 +0100
	    Implement remote_pid_to_exec_file using qXfer:exec-file:read


Thanks,
Jan

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gdb/ChangeLog
2016-03-19  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* remote.c (remote_pid_to_exec_file): Print warning for unsupported
	PACKET_qXfer_exec_file.

diff --git a/gdb/remote.c b/gdb/remote.c
index af0a08a..d267736 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -12977,7 +12977,13 @@ remote_pid_to_exec_file (struct target_ops *self, int pid)
   char *annex = NULL;
 
   if (packet_support (PACKET_qXfer_exec_file) != PACKET_ENABLE)
-    return NULL;
+    {
+      warning (_("No executable has been specified (see the \"file\" command) "
+                 "and remote gdbserver does not "
+		 "support packet \"qXfer:exec-file:read\""
+		 " - please use FSF gdbserver version 7.10 or later."));
+      return NULL;
+    }
 
   if (filename != NULL)
     xfree (filename);