ls(1): Fix sort-by-size bug

ls(1): Fix sort-by-size bug

For a reason that escapes me, in order to sort files by size using ls(1),
one had to pass both the size sort switch (-S) and the time sort switch
(-t), otherwise the -S flag would be ignored.  This hacky workaround was
documented in the man page as well.

Let's fix this with code taken from FreeBSD.  The -t and -S switches are
now mutually exclusive.  If someone uses them together, then the last
one wins (e.g. ls -st sorts by mod. time and -ts sorts by file size)