Handle strace files without pid

[mgedmin]

In mgedmin/check-manifest#141 (comment) there's a link to a gist 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

<timestamp> clone(strace: Process NNN attached
<args...>

lines to read

<timestamp> clone(<args...>

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