[gdb/cli] Add ignore-errors command

While trying to reproduce a failing test-case from the testsuite on the
command line using a gdb command script, I ran into the problem that a command
failed which stopped script execution.

I could work around this by splitting the script at each error, but I realized
it would be nice if I could tell gdb to ignore the error.

A python workaround ignore-errors exists, mentioned here (
https://sourceware.org/legacy-ml/gdb/2010-06/msg00100.html ).  This also
mentions a bugzilla entry that adds "exception handling to the gdb command
language" but I was not able to find that.

This patch adds native ignore-errors support (so no python needed).

So with this script:
...
$ cat script.gdb
ignore-errors run
echo here
...
we have:
...
$ gdb -q -batch -x script.gdb
here$
...

We could also implement this as first-class command, like so:
...
static void
ignore_errors_command (const char *args, int from_tty)
{
  try
    { execute_command (args, from_tty); }
  catch (const gdb_exception_error &ex)
    { async_enable_stdin (); }
}
...
and:
...
  add_cmd ("ignore-errors", class_support, ignore_errors_command,
           source_help_text, &cmdlist);
...
but that means we go twice through execute_command.  This seems cleaner, at
least in that aspect.

---
 gdb/event-top.c | 29 +++++++++++++++++++++++++++--
 1 file changed, 27 insertions(+), 2 deletions(-)

diff --git a/gdb/event-top.c b/gdb/event-top.c
index 002a7dc95e0..9d36b550dda 100644
--- a/gdb/event-top.c
+++ b/gdb/event-top.c
@@ -583,13 +583,38 @@ command_handler (const char *command)
   /* Do not execute commented lines.  */
   for (c = command; *c == ' ' || *c == '\t'; c++)
     ;
-  if (c[0] != '#')
+  if (c[0] == '#')
+    return;
+
+  /* Detect and skip "ignore-errors". */
+  const char * s = "ignore-errors";
+  const size_t s_len = strlen (s);
+  bool ignore_errors_p
+    = (strncmp (c, s, s_len) == 0
+       && strlen (c) > s_len
+       && (c[s_len] == ' ' || c[s_len] == '\t'));
+  if (ignore_errors_p)
     {
-      execute_command (command, ui->instream == ui->stdin_stream);
+      c += s_len;
+      for (; *c == ' ' || *c == '\t'; c++)
+	;
+    }
+
+  try
+    {
+      execute_command (c, ui->instream == ui->stdin_stream);
 
       /* Do any commands attached to breakpoint we stopped at.  */
       bpstat_do_actions ();
     }
+  catch (const gdb_exception_error &ex)
+    {
+      if (!ignore_errors_p)
+	throw;
+
+      /* See also execute_gdb_command.  */
+      async_enable_stdin ();
+    }
 }
 
 /* Append RL, an input line returned by readline or one of its