Ignore dotfiles when discovering magic files

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.

Fix custom output directory for extracted files

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.

Improved support for DIR100D1/E1 firmware files

deps.sh: replace a leftover "sudo" with "$SUDO"

Added comment to extract.conf noting the use of an internal plugin for Hilink encrypted firmware extraction.

Fixed extraction bugs when recursively extracting multiple files at once; added some extraction API methods.

deps: check for package installation failure