Copy the recent improvements from the FreeBSDRemote plugin, notably:
- moving event reporting setup into SetupTrace() helper
- adding more debug info into SIGTRAP handling
- handling user-generated (and unknown) SIGTRAP events
- adding missing error handling to the generic signal handler
- fixing attaching to processes
- switching watchpoint helpers to use llvm::Error
- minor style and formatting changes
This fixes a number of tests, mostly related to fixed attaching.