m_option: fix braindead --start, --length, --end option parsing bugs

The option type m_option_type_rel_time was completely broken. It
interpreted everything starting with a number as percent position. This
is because sscanf() semantics are idiotic (trailing string doesn't need
to be matched), and due to my own idiocy this was overlooked when
testing. Fix by considering sscanf() evil and not using it.
(bstr_sscanf() is a straight wrapper around sscanf()).

Even if the percent code was fixed, there was another bug: it always
interpreted times as negative (starting from end for --start). Fix the
basic logic.