loader(8): Update: add more text on kenv vs local variables etc.
- Add missing kenv variables to assignment white list;
all kenv variables which are used in loader(8) source are added.
- Inhibiting loading of acpi(4) needs to use `lunset acpi_load',
not `unset', as `loadall' uses local variables; ditto for ehci.
- Add info to loader.8 & help files on variables ($VAR / ${VAR}) in
command arguments and assignment; these are kenv variables.
- Add description of vfs.root.realroot
- Fix some whitespace style(7).
DragonFly-bug: <http://bugs.dragonflybsd.org/issue2141>