Replace sscanf() by strtoul()

This change has been motivated by a mail from bde sent in 2015 in
which he mentioned inappropriate use of sscanf() in 3 programs in
/bin.

This change removes the potential mismatch of the types of the return
values and the variable width specified in the scan pattern. While
there was no issue with the patterns and types used, the new code is
simpler and more efficient.