install header files required development with libzfs_core

libzfs_core provides a rather limited but committed (stable) interface
for working with ZFS. We install libzfs_core shared library but we do
not install header files required for developing programs that use
the library. This change is to install the required header files
libzfs_core.h, libnvpair.h and sys/nvpair.h.

The headers are installed into the same locations as on illumos.

Reviewed by: mav, markj
Differential Revision: https://reviews.freebsd.org/D8005