Put section and architecture info into the keys table,

Put section and architecture info into the keys table,
in preparation for removing them from the mpages table,
aiming for cleaner and more uniform interfaces.
Database growth is below 4%, part of which will be reclaimed.

As a bonus, this allows searches like:
./obj/apropos An=kettenis -a arch=ppc
./obj/apropos An=kettenis -a sec~[^4]