Various cleanups

Various cleanups to the code:

* Faster copyString by reading entire words at a time from child memory
* Closing all unused FDs on exec
* Handling SIGABRT as a cue to launch the debugger
* Adding *nanosleep, madvise and exit_group to syscall whitelist
* Printing a message when a process is killed by SIGSYS as a result of using disallowed syscalls
* Starting the signal handler after opening sockets to delay that action as long as possible
* Fixing overwriting beyond a child's buffer in writeData