Switch to ascii armored sig files

Many years ago my gpg was defaulting to unarmored .sig files, but that's
not as friendly as the armored .asc files. We've been meaning to change
for a while but wanted to keep to my de-facto standard in 3.8 and 4.0.
4.2 seems like a good place to switch.
Merge branch '4.2/clearsign-snapshots'