Misc cleanups
	Fix declaration of zfs_destroy_snaps_nvl in libzfs.h.
	Replace the zfs_snaps_nvl function in toto from Illumos.
	Fix the Linux-specific code in zfs_snapshot [NOTE: Check the XXXTSC comment].
	A few un-C99s.
	Fix an initializer of a zfs_cmd_t in libzfs_sendrecv.c.
libzfs_core: Fix assignments of NULL to zc_nvlist_dst (it's now a 64-bit integer).
configure.ac: Add Makefile for libzfs_core