Keep track of the last active slot in the shared ProcState array, so

Keep track of the last active slot in the shared ProcState array, so
that search loops only have to scan that far and not through all maxBackends
entries.  This eliminates a performance penalty for setting maxBackends
much higher than the average number of active backends.  Also, eliminate
no-longer-used 'backend tag' concept.  Remove setting of environment
variables at backend start (except for CYR_RECODE), since none of them
are being examined by the backend any longer.