/usr/libexec is for programs which must be started some strange way,
which the user could not meaningfully start from the command line.
mount_* can all be run just fine from the command line; it just so
happens that one usually runs them through `mount'.  Most of the
things in /usr/libexec and started from inetd, and would not work from
the command line (since they do things like getsockname (0, ...)).

This rationale does not explain why cc1 et al are there.  I guess they
are just things you "would never want" to run by hand.

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