Install symlinks to individual headers instead of symlinks to directories

Install symlinks to individual headers instead of symlinks to directories
in the SHARED=symlinks case.  Symlinks to directories only work if all the
the necessary headers are in 1 directory, but the necessary headers are
scattered for at least ipfilter headers in <netinet>.  This change also
avoids polluting /usr/include with non-headers; the /usr/include hierarchy
is now independent of the setting of SHARED.

Submitted by:	ru (edited to fix netgraph/bluetooth/include and machine/pc)
PR:		44148