* progmodes/gdb-mi.el (gdb-init-1): Disassembly buffer mode name

* progmodes/gdb-mi.el (gdb-init-1): Disassembly buffer mode name
may contain frame information, so `string-match' should be used.
(gdb-update): Disassembly is invalidated through
`gdb-get-selected-frame'.
(gdb-pad-string): New function to pad string with spaces.
(gdb-invalidate-disassembly): Invalidate only if the buffer
exists.
(gdb-disassembly-handler-custom): Column alignment.
(gdb-disassembly-place-breakpoints): Clear old breakpoints before
placing new ones.
(gdb-toggle-breakpoint, gdb-delete-breakpoint): Now work from the
end of line, too.
(gdb-frame-handler): Match convention to for disassembly buffer
mode name.