extractor: Improved warning for when the external extractor is not found, for issues such as devttys0/binwalk#245

Removed excessive size field validation for LZMA signatures; their compression is already validated, which is a better metric anyway.

Update kwarg processing to ensure modules don't receive a mutable reference to a class variable default. Fixes #207, also the Signature Modulemodule was leaking memory.

Some programs (which?) might create temporary files like `~name` or
`_name`, but maybe there are people who actually rely on names like
`_name`. Let's restrict the filter to just dotfiles for now.

Fixes regression since 5fdad3c5 which
ignored the -C option and wrote all files in the current working
directory.

Tested with:

    touch foo && gzip foo && mkdir bar
    binwalk -e foo.gz -C bar
    binwalk -e foo.gz

I was a bit surprised to get an "UnicodeDecodeError: 'utf-8' codec can't
decode byte 0xbe in position 21: invalid start byte" error while
writing and testing a new magic file.

It turns out that binwalk tried to parse the Vim swap file (.magic.swp)
as a magic file which obviously fails. Therefore ignore all files not
starting with an alphanumeric character.