Link libshare directly to libzfs

Link libshare directly to libzfs

Drop usage of dlopen/dlsym for libshare.  There is no need to do
this because the zfs packages provide libshare.  Unlike on Solaris
we are guaranteed it will be available.

This avoids possible problems with hardcoding the libshare path in
the code (e.g. when users specify a different install path via
configure options).  It additionally simplifies the code which is
good for maintainability.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>