apropos commands for commands, user options, all functions, all variables

[Drew Adams]

I think we should do the following (I do it in my own code):

* Rename `apropos-variable' to `apropos-option'.

* Define a new `apropos-variable' command that works for all variables, not
just user options.

* Define an `apropos-function' command that works for all functions, not
just commands.

Users should be able to find and use commands that give them info about (a)
all vars, (b) all functions, (c) just commands, and (d) just user options;
and the names of those commands should reflect what they do.


Similarly, we should rename `set-variable' to `set-option' and define a new
`set-variable' command that sets any variable, not just a user option. (I
have defined such a command in my own code, but I just call it
`set-any-variable', for now.)