index: update [extindex "all"] by default, support -E

index: update [extindex "all"] by default, support -E

In most cases, this ensures users will only have to opt-in to
using -extindex once and won't have to issue extra commands
to keep external indices up-to-date when using
public-inbox-index.

Since we support arbitrary numbers of external indices for
ease-of-development, we'll support repeating "-E"
("--update-extindex=") in case users want to test changes in
parallel.