Strace-process-tree

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* PEP-8 and slight readability refactoring.

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* New strace in Ubuntu 18.04 LTS formats its log files differently.
* Recognize fork().

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* Use argparse, add help message.
* Better error reporting.
* Print just the command lines instead of execve() system call arguments
(pass -v/--verbose if you want to see full execve() calls like before).
* execve() is more important than clone().
* Distinguish threads from forks.
* This was the last version released as a Gist. Newer versions were available
from `my scripts repository
<https://github.com/mgedmin/scripts/blob/master/strace-process-tree>`__.

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* Strip trailing whitespace in output.

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* Handle strace -T output.
* Simplify clone() args in output.

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* Support vfork() and fork().