back to using grep fixed-string matching when removing key lines

back to using grep fixed-string matching when removing key lines

This method uses grep -F to find the full line to match, and then
second call to grep -v -F to actually remove the line.  For
known_hosts, we use two piped grep -F calls.  No rexexp are used, and
only one extra call to grep is required for known_hosts line removal.

There is still an issue here about sub-string matches, but there is at
least no regression over early versions.