Replacing header patterns with signature patterns was a mistake, since
both are quite different from each other, and both have their uses. In
fact, this caused a serious regression: for such files as shell scripts
without *.sh extension but with #!/bin/sh inside, filetype detection
does not work at all anymore.
Since both header and signature patterns are useful, reintroduce support
for header patterns while keeping support for signature patterns as well
and make both work nicely together.
Also, unlike in the old implementation (before signatures were
introduced), ensure that filename matches take precedence over header
matches, i.e. if there is at least one filename match found, all header
matches are ignored. This makes the behavior more deterministic and
prevents previously observed issues like #2894 and #3054: wrongly
detected filetypes caused by some overly general header patterns.
Precisely, the new behavior is:
1. if there is at least one filename match, use filename matches only
2. if there are no filename matches, use header matches
3. in both cases, try to use signatures to find the best match among
multiple filename or header matches
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