sysutils/pot: Update to 0.15.4

Includes adding a new group "pot" which is used to grant
unprivileged users access to read-only pot commands.

Depend on security/signify for pot image signatures.

While there, add a stub man page so users entering man pot see
something slightly useful, readd pkg-message.in to package.

Add a longer pkg-descr to make portlint happy.

Approved by: pizzamig (maintainer)