pager: drop the support for 'pager.enable=<bool>' This option was never released except for a release candidate. Dropping compatibility with this option will free the 'pager.enable' config option for other usage in the future.

pager: rename 'pager.enable' to 'ui.paginate' This aligns with what we do for color (see 7fec37746417). Pager is a central enough notion that having the master config in the [ui] section makes senses. It will helps with consistency, discoverability. It will also help having a simple and clear example hgrc mentioning pager. The previous form of the option had never been released in a non-rc version but we keep it around for convenience. If both are set, 'ui.pager' take priority.

color: special case 'always' in 'ui.color' This lift the confusing case, where 'ui.color=always' would actually not always use color.