- the jtag chain is examined and validated after GDB & telnet servers

- the jtag chain is examined and validated after GDB & telnet servers
 are up and running. The examination and validation is actually
 "optional" from the point of view of GDB + telnet servers.
 Multiple targets should work fine with this.
- jtag_speed is dropped(divisor is increased), if jtag examination and
 validation fails.
- the chain is validated 10x to catch the worst jtag_speed offences
- added LOG_SILENT that can be used to shut up log. Feeble
 ersatz for try+catch.
- GDB register packets are now always replied in order to make sure
 that GDB connect works. If the target is not halted, then these
 packets contain dummy values.

git-svn-id: svn://svn.berlios.de/openocd/trunk@483 b42882b7-edfa-0310-969c-e2dbd0fdcd60