FreeBSD ProcessMonitor: map TRAP_CAP to a trace trap

In the absense of a more specific handler for TRAP_CAP (generated by
ENOTCAPABLE or ECAPMODE while in capability mode) treat it as a trace
trap. Obtained from FreeBSD r318884.

We should later add an option to have LLDB control the trapcap procctl 
(as with ASLR), as well as report a specific stop reason. For now this
change eliminates an assertion failure from LLDB.

llvm-svn: 303965