Singificantlly improve message scan pointer initialization (e.g. for new

users) and adjusting scan ptrs by date or set to "Last msg" while online:
- speed (on my system, reduced from 12 to < 2 seconds)
  This involved 2 enhancements: check msgbase index file timestamp (fast)
  before opening any files. When opening the msgbase, just open the index
  file (no other msgbase files) - required new smblib function: smb_open_index.
- fixed off-by-one when setting by date (when setting the new-scan ptr to
  Jan-1-2018, you want that first message posted on/after Jan-1-2018 to be
  found as "new", not the first message posted *after* that date.
- re-added the fixmsgptrs() call in putmsgptrs(), inadvertently removed
  in the .ixb -> .ini ptr storage update in rev 1.192 of userdat.c.