(gud-debugger-startup): Replaced with gud-massage-args.

(gud-debugger-startup): Replaced with gud-massage-args.
(gud-{gdb,sdb,xdb}-debugger-startup): Replaced with gud-*-massage-args.
(gdb, sdb, xdb): Argument is command line, not args for it.  Remove
debugger name from prompt and put it in the default input instead.
Overload gud-massage-args instead of gud-debugger-startup.
(gud-chop-words): New function; subr for gud-common-init.
(gud-common-init): Argument is command line, not args for it.
Rewritten to use gud-chop-words instead of temp buffer and kludge involving
read.  Find the program name as the first word of the command line.
Use make-comint and gud-massage-args in place of gud-debugger-startup.
Expand the file name before passing it to gud-massage-args.