scansys.py: allow setting tags from the command line.
Revise scansys.py to compute the exact set of prereqs for each header.

Ancillary changes:
 * datafile format now has '$' lines which are annotations for the
   immediately previous header, intended to be copied into the
   presentation table.
 * This is used to mark special headers (automatically) and buggy headers
   (with human editing) as well as for prereqs.
 * datafile format now has a ':gen N' label indicating the last major
   update to scansys, so we can easily identify inventories that are
   in need of regeneration.
 * Regenerate glibc2.13 annotations with the above changes.