Abstract sysop user status using userdat.c function and new sbbs_t method

... instead of C macros: SYSOP and REALSYSOP

REALSYSOP -> user_is_sysop(&useron)
SYSOP -> sbbs_t::useron_is_sysop()

This change also also includes a bug fix in chk_ar.cpp:
sbbs_t::chk_ar() would evalulate the ARS keyword "SYSOP" as being true when
temporary sysop status was set (e.g. after CHUSERing to another user) even
when the user being checked was not the current user online (useron). The only
obvious impact of this bug (that I found) would be when using the bulkmail
feature and wanting to send to only SYSOPs *after* having CHUSER'd to another
user, it would actually send bulk mail to *all* users. Listing users with
access to a particular message or file area that had an access restriction
of "SYSOP" (using sbbs_t::userlist()) would also have this issue (it would
list *all* users). ARS searches in the online user editor for "SYSOP" would
also do weird stuff in this case.