RFA Fix regressions for multi breakpoints command line setting/clearing

[Philippe Waroquiers <philippe.waroquiers@skynet.be>]

Fix regressions for multi breakpoints command line setting/clearing

Fix done in breakpoint.c
gdb.base/commands.exp changed to reproduce/test the problems.


Note that these regressions appeared in GDB 8.1, e.g.
  GNU gdb (GDB) 8.1
  ...
  (gdb) command 1 2
  Type commands for breakpoint(s) 1 2, one per line.
  End with a line saying just "end".
  >p     987654321
  >end
  No breakpoint number 54321.
  ...
  (gdb) command 1 2
  Type commands for breakpoint(s) 1 2, one per line.
  End with a line saying just "end".
  >end
  Type commands for breakpoint(s) 1 2, one per line.
  End with a line saying just "end".
  >end
  (gdb)

So, wondering if this fix should not be merged back
to 8.2 (and/or in 8.1.2) ?