Rationalize "backtrace" command line parsing

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.