Needed to bump it to 16 anyway (the RFC says 15 plus the protocol command itself, so 16), but EPIC5 bumped it
up to 20 based on a report from users of RusNet so we might as well go up to that as well.
Tokens that begin with two underscores __* or an underscore and an uppercase letter _X* are reserved,
so we should avoid those for our own include guards. The standard I'm settling on for foo.h is FOO_H_.
