[lldb] [Process/FreeBSDRemote] Fix handling user-generated SIGTRAP

Update the SIGTRAP handler to account for the possibility of SIGTRAP
being generated by the user, i.e. not having any specific debugging
event associated with it, as well as receiving unknown SIGTRAPs.  These
instances of SIGTRAP are passed to the regular signal handler.

Differential Revision: https://reviews.llvm.org/D91007