[File] [PATCH] fix accidental 'any char' regexps

[Dirk Müller]

In several occassions the regexps were using '\.' when '\\.'
was intended, as the libmagic parsing does one level of dequoting.

This is important as it causes "any char" to match rather than ".", and
makes the regexps fall to backtracking where not needed. So avoiding
that makes things more performant and more correct.

Also when we're returning version numbers, search for nonempty matches.
Add sanity check during compile step that prints a warning.
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