COPYTREE_SHARE should also be using _SHAREMODE and not SHAREMODE.

SHAREMODE defaults to 444, and gives us endless grief because one
cannot modify a file that is not writable.
So use _SHAREMODE that is 644, so that we can avoid all those CHMOD +w
in so many Makefiles.

Exp-run: antoine
Sponsored by: Absolight
Differential Revision: https://reviews.freebsd.org/D7623