[krbdev.mit.edu #1208] Provide install-headers target

[Sam Hartman via RT]

KFM wants an install-headers target to install the public headers that
would be installed  at install time.

For now, I'm providing a target install-headers (which calls
install-headers-unix after recursion) that installs all the public
headers.

I suspect there may be differences between what KFM wants *int terms
of actual headers installed and what we currently do.  When this turns
out to be the case we should discuss whether we change what we're
doing or fix up using jam after calling install-headers to a temporary
directory.