Moved flag processing as far "inward" as possible when dealing with
compile-stem, reducing the amount of redundant code for parsing out and
passing along boolean keywords based on the presence or absence of a
flag and eliminating some of the keyword arguments to compile-stem.
Added a "mode" parameter to compile-stem to enable determining the
correct compile-file function based on the combination of mode and
flags, further simplifying the interface.
Added new functions for determining the source and object pathnames
for a stem, fixing a longstanding KLUDGE in host-load-stem,
consolidating the three instances of code to compute an object pathname
and the two instances of code to compute a source pathname and
eliminating the rest of the keyword arguments to compile-stem.