[attributes] Allow boolean, single-value attributes.

Following discussion in https://github.com/coq/coq/pull/12586 , we
extend the syntax `val=string` to support also arbitrary values.

In particular we support boolean ones, or arbitrary key-pair lists.

This complements the current form `val="string"`, and makes more sense
in general.

Current syntax for the boolean version is `attr=yes` or `attr=no`, but
we could be more liberal if desired.

The general new guideline is that `foo(vals)` is reserved for
the case where `vals` is a list, whereas `foo=val` is for attributes
that allow a unique assignment.

This commit only introduces the support, next commits will migrate
some attributes to this new syntax and deprecate the old versions.

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