Improved documentation of feature dependencies by Fare, editing by RPG.
Minor nits that makeinfo didn't like.
Putting introspection functions in canonical form.

SYSTEM-DEFSYSTEM-DEPENDS-ON, SYSTEM-DEPENDS-ON, and
SYSTEM-WEAKLY-DEPENDS-ON now always return s-expressions with system
names represented as downcased caps (per COERCE-NAME).
Bump version to 3.1.0.72
Some tweaks to the documentation.
Docstring for REQUIRE-SYSTEM.
Check for use of obsolete "FEATURE" syntax in dependencies.

The dependency parser (used to put dependencies in canonical form), now
raises an error if obsolete FEATURE syntax is used. This should direct
programmers to an easy fix.

A test is also added.
Undo 824c0f0 from Feb 22, and leave a comment why the thing is working.
Avoid an error-in-warning while testing for whether a system is builtin:
the target system for a version check might not even be present!
(That would probably be an error later, to be handled then,
but shouldn't cause an unhandled error now.)
Bump version to 3.1.0.73