refactor stream.h; fix some issues with last patch

Most importantly, the last patch for imfile contained a number
of glitches, which are fixed by this commit (a memory leak under
unusual conditions, partial message loss when rsyslog was
terminated in the interim & mixing file data to the wrong files
when multiple monitors were used [due to static variable]). The
commit is actually a re-write of the patch, based on its core
idea.

Also some other minor cleanup was done.

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