loader(8): Update: add more text on kenv vs local variables etc.

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>