lldb: Threaded inferior support for FreeBSD

This is in the process of being submitted to the upstream LLDB
repository.  The thread list functionality is modelled in part on
GDBRemoteCommunicationClient.

LLDB bug pr16696 and code review D2267

Sponsored by:	DARPA, AFRL
Add a new sysctl / loader tunable kern.panic_reboot_wait_time which
defaults to PANIC_REBOOT_WAIT_TIME (a long-existing kernel config
setting).  Use this now-variable value in place of the defined constant
to control how long the system waits after a panic before rebooting.