DTrace(FreeBSD) record support

Currently samply does NOT support `record` in FreeBSD - will need to use `dtrace` for this. 

```sh
#!/bin/bash
$@ &
TRACE_PROBE="profile-397 /pid == $! && arg1/ { @[ustack()] = count(); } tick-60s { exit(0); }"
sudo dtrace -Z -n "$TRACE_PROBE" -o dtrace-out.user_stacks 2>/dev/null || exit
```
This kind of "remote invokation" works usually... but libdtrace exists.
