libbe: lift the WARNS post-OpenZFS merge

sys/ccompile.h no longer uses #pragma ident, so we no longer need to worry
about unknown pragmas.

I fixed one WARNS issue in r363409 by annotating be_is_auto_snapshot_name's
lbh parameter __unused, then upstreamed the following changes to OpenZFS
that rode in with the merge:

• zfs_path_to_zhandle now takes a const char *path rather than a char *path, since it won't be mutating the string it receives and I had no reason to believe it will need to in the future. [OpenZFS PR #10605]
• Annotated some unused parameters on definitions inlined into headers as such. [OpenZFS PR #10606]