Rationalize "backtrace" command line parsing

The backtrace command has peculiar command-line parsing.  In
particular, it splits the command line, then loops over the arguments.
If it sees a word it recognizes, like "full", it effectively drops
this word from the argument vector.  Then, it pastes together the
remaining arguments, passing them on to backtrace_command_1, which in
turn passes the resulting string to parse_and_eval_long.

The documentation doesn't mention the parse_and_eval_long at all, so
it is a bit of a hidden feature that you can "bt 3*2".  The strange
algorithm above also means you can "bt 3 * no-filters 2" and get 6
frames...

This patch changes backtrace's command line parsing to be a bit more
rational.  Now, special words like "full" are only recognized at the
start of the command.

This also updates the documentation to describe the various bt options
individually.

gdb/ChangeLog
2018-03-26  Tom Tromey  <tom@tromey.com>

	* stack.c (backtrace_command): Rewrite command line parsing.

gdb/doc/ChangeLog
2018-03-26  Tom Tromey  <tom@tromey.com>

	* gdb.texinfo (Backtrace): Describe options individually.

2 files changed, 33 insertions, 31 deletions

diff --git a/gdb/doc/ChangeLog b/gdb/doc/ChangeLog
index 2441f15..413e456 100644
--- a/gdb/doc/ChangeLog
+++ b/gdb/doc/ChangeLog
@@ -1,3 +1,7 @@
+2018-03-26  Tom Tromey  <tom@tromey.com>
+
+	* gdb.texinfo (Backtrace): Describe options individually.
+
 2018-03-19  Tom Tromey  <tom@tromey.com>
 
 	* observer.texi: Remove.
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index 74e0fdb..28254c9 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -7307,45 +7307,43 @@ frame (frame zero), followed by its caller (frame one), and on up the
 stack.
 
 @anchor{backtrace-command}
-@table @code
 @kindex backtrace
 @kindex bt @r{(@code{backtrace})}
-@item backtrace
-@itemx bt
-Print a backtrace of the entire stack: one line per frame for all
-frames in the stack.
-
-You can stop the backtrace at any time by typing the system interrupt
-character, normally @kbd{Ctrl-c}.
-
-@item backtrace @var{n}
-@itemx bt @var{n}
-Similar, but print only the innermost @var{n} frames.
-
-@item backtrace -@var{n}
-@itemx bt -@var{n}
-Similar, but print only the outermost @var{n} frames.
-
-@item backtrace full
-@itemx bt full
-@itemx bt full @var{n}
-@itemx bt full -@var{n}
-Print the values of the local variables also.  As described above,
-@var{n} specifies the number of frames to print.
-
-@item backtrace no-filters
-@itemx bt no-filters
-@itemx bt no-filters @var{n}
-@itemx bt no-filters -@var{n}
-@itemx bt no-filters full
-@itemx bt no-filters full @var{n}
-@itemx bt no-filters full -@var{n}
+To print a backtrace of the entire stack, use the @code{backtrace}
+command, or its alias @code{bt}.  This command will print one line per
+frame for frames in the stack.  By default, all stack frames are
+printed.  You can stop the backtrace at any time by typing the system
+interrupt character, normally @kbd{Ctrl-c}.
+
+@table @code
+@item backtrace [@var{args}@dots{}]
+@itemx bt [@var{args}@dots{}]
+Print the backtrace of the entire stack.  The optional @var{args} can
+be one of the following:
+
+@table @code
+@item @var{n}
+@itemx @var{n}
+Print only the innermost @var{n} frames, where @var{n} is a positive
+number.
+
+@item -@var{n}
+@itemx -@var{n}
+Print only the outermost @var{n} frames, where @var{n} is a positive
+number.
+
+@item full
+Print the values of the local variables also.  This can be combined
+with a number to limit the number of frames shown.
+
+@item no-filters
 Do not run Python frame filters on this backtrace.  @xref{Frame
 Filter API}, for more information.  Additionally use @ref{disable
 frame-filter all} to turn off all frame filters.  This is only
 relevant when @value{GDBN} has been configured with @code{Python}
 support.
 @end table
+@end table
 
 @kindex where
 @kindex info stack