Completion insecurities handled on startup.

Completion-dependent directories with insecure permissions or ownership (e.g.,
group or other writability) are now detected, reported, and sanitized on OMZ
startup rather than unsafely ignored -- which resulted in effectively empty
completion caches and obscure compdef() errors resembling
"compdef: unknown command or service: git".

This fixes long-standing issues #630, #3356, and #3455 and related Babun issues
159, 281, and 322 -- and probably numerous other duplicates.