gitattributes: introduce and use "generated" attribute

gitattributes: introduce and use "generated" attribute

I want to mark some files to be ignored for licensing purposes,
e.g. output from fuzzers and other samples. By using the gitattribute
machinery for this we don't need to design a custom protocol:

$ git check-attr generated test/test-sysusers/unhappy-*
test/test-sysusers/unhappy-1.expected-err: generated: set
test/test-sysusers/unhappy-1.input: generated: unspecified
test/test-sysusers/unhappy-2.expected-err: generated: set
test/test-sysusers/unhappy-2.input: generated: unspecified
test/test-sysusers/unhappy-3.expected-err: generated: set
test/test-sysusers/unhappy-3.input: generated: unspecified