cmd/etrace: add --use-flatpak-run option to exec

[anonymouse64]

Needs to be researched a bit for the file command, but it works well for the
exec command using the automatic classname option with xdotool.

Also add a corresponding option for --classname.

Example output with a flatpak:

$ ./etrace exec --use-flatpak-run --no-trace org.gabmus.whatip
Total startup time: 1.596067141s
Gdk-Message: 17:49:30.377: org.gabmus.whatip: Fatal IO error 0 (Success) on X server :99.0.

It still needs some love when it comes to tracing, as it just outputs /bin/sh for some reason right now:

$ ./etrace exec -f org.gabmus.whatip

Gdk-Message: 17:49:08.685: org.gabmus.whatip: Fatal IO error 0 (Success) on X server :99.0.

1 exec calls during snap run:
     Start  Stop   Elapsed      Exec
     0      19078  19.078016ms  /bin/sh
Total time:  2.246505976s
Total startup time: 2.233035943s

Updates #25