Instead of `config <pkgbase> keywords <keyword1> <keyword2>...`, the new
syntax is `set-keywords <pkgbase> <keyword1> <keyword2>...`.

The `config` keyword was rather general and it was not obvious that it
could be used to change package base settings. Instead of replacing it
with an even more verbose expression, remove that unnecessary level of
indirection. Since we do not (and probably never will) support keywords
anywhere else, the chance of name conflicts is small.

Suggested-by: Dave Reisner <dreisner@archlinux.org>
Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>