(struct stab_link_includes_totals): Rename field 'total' to 'sum_chars'

(struct stab_link_includes_totals): Rename field 'total' to 'sum_chars'
and add field 'num_chars'.
(_bfd_link_section_stabs): When computing the sum of the characters in a
B_INCL..B_EINCL range also keep a count of the number of characters.  Use
this information to help distinguish between include sections when have
the same sum but which nevertheless are still unique.