large file support

[adrelanos]

Created a 2G file for test purposes.

fallocate -l 2G test.img

Tried to sign it.

signify-openbsd -S -s ./keyname.sec -m ./test.img -x ./test.img.sig

result:

signify-openbsd: msg too large in ./test.img

Could you please add support for signing arbitrarily large files?