accesslib: Introduce (and use) reload_all_capabilities()

accesslib: Introduce (and use) reload_all_capabilities()

If accessinfo is stale, we need to reload it without losing
out "interesting" state -- transparently for the end user.

That means preserving active role switches, loginas (site and course
level), etc. The logic for that is encapsulated in
reload_all_capabilities().

Also affected:
 - has_capability() which now calls reload_all_capabilities()
 - role_switch() - minor tidyup