Handle strace files without pid

In https://github.com/mgedmin/check-manifest/issues/141#issuecomment-819457988 there's a link to [a gist](https://gist.githubusercontent.com/jugmac00/ee8068afa2f30be57de44401e7ab474e/raw/ddde3157df4567ca9aca3f6f89032ce35b9713fc/out.txt) containing an strace file that strace-process-tree refuses to parse. It has entries without a pid, followed by entries with a pid, and eventually going back to entries without a pid.

It is parseable after a small massaging: find a `getpid() = NNN` line to learn the pid of the main process, prepend `[pid NNN]` to all the lines that don't have a `[pid ...]` prefix, and fixup all the

clone(strace: Process NNN attached

lines to read

clone(

There's no reason strace-process-tree can't do these things itself.

Start with the strace-process-tree input parsing entry point and reproduce the failure using the linked gist. The work is done when traces with entries both with and without pid prefixes, including the shown clone attachment lines, are parsed successfully.

Written by the indexing model from the issue text.