execsnoop: ensure tracing_on is enabled during execution

[dannyk81]

Huge fan here! your tools are so useful and I learn a lot from the code itself so thanks for making them and putting them here for all of us to enjoy.

This is just a small adjustment to set tracing_on to 1 (and disable it when done) in execsnoop, perhaps there was a good reason not to have this in place, but this helped me get some results 😄

if this is not a good idea, maybe a note of this in the help section/README.

fixes #29

[brendangregg]

thanks; I commented in #29: we need to know why tracing_on is becoming 0 before forcing it to 1.