grep: support -e, -f, multiple patterns in general

[klange]

Glossing over the inabilities of our regex engine, our grep is now very close to full POSIX compliance, it's just missing these options for providing multiple patterns:

• -e doesn't really do anything, it just takes the place of the implicit pattern argument, and can be specified multiple times to provide multiple sets of patterns.
• -f reads patterns from a file.
• The pattern argument, and -e, and also how files should be read... all allow multiple patterns separated by line feeds, which should act as an OR match.

[klange]

I think we can manage all of this in subsearch_matches. Since POSIX grep does not care about multiple matches on a line, we don't need to think too much about which match we continue searching with unless we really want to have equivalent output to GNU grep in color mode.