- Improves error handling upon GDB connect

- Improves error handling upon GDB connect
- switch to synchronous halt during connect. This fixes the bug
  where poll() was not invoked between halt() and servicing the
  'g' register packet
- halt() no longer returns error code when target is already halted, just
  logs a warning. Only the halt() implementation can say anything
  meaningful about why a halt() failed, so error messages are pushed
  up to halt()
- fixed soft_reset_halt infinite loop bug in arm7_9_common.c. The rest
  of the implementations are still busted.
- by using USER() instead of command_print() the log gets the
  source + line #. Nice.
- no longer invoke exit() if soft_reset_halt fails. A reset can often
  fix the problem.

git-svn-id: svn://svn.berlios.de/openocd/trunk@475 b42882b7-edfa-0310-969c-e2dbd0fdcd60