Simplify handle_sleep_command:

- Use new parse_ulong to ensure duration parses as a valid number.
- Rework logic to improve readability and seliminate uperfluous braces.
- Change whitespace to improve style.


git-svn-id: svn://svn.berlios.de/openocd/trunk@2207 b42882b7-edfa-0310-969c-e2dbd0fdcd60