Define an attribute for each crypto algorithm, and use that attribute
to select the files that implement the algorithm.
* Give the "wlan" attribute a dependency on the "arc4" attribute.
* Give the "cgd" pseudo-device the "des", "blowfish", "cast128", and
"rijndael" attributes.
* Use the new attribute-as-option-dependencies feature of config(8) to
give the IPSEC_ESP option dependencies on the "des", "blowfish", "cast128",
and "rijndael" attributes.
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