MFOpenZFS: Add basic zfs ioc input nvpair validation

We want newer versions of libzfs_core to run against an existing
zfs kernel module (i.e. a deferred reboot or module reload after
an update).

Programmatically document, via a zfs_ioc_key_t, the valid arguments
for the ioc commands that rely on nvpair input arguments (i.e. non
legacy commands from libzfs_core). Automatically verify the expected
pairs before dispatching a command.

This initial phase focuses on the non-legacy ioctls. A follow-on
change can address the legacy ioctl input from the zfs_cmd_t.

The zfs_ioc_key_t for zfs_keys_channel_program looks like:

static const zfs_ioc_key_t zfs_keys_channel_program[] = {

{"program",     DATA_TYPE_STRING,               0},
{"arg",         DATA_TYPE_UNKNOWN,              0},
{"sync",        DATA_TYPE_BOOLEAN_VALUE,        ZK_OPTIONAL},
{"instrlimit",  DATA_TYPE_UINT64,               ZK_OPTIONAL},
{"memlimit",    DATA_TYPE_UINT64,               ZK_OPTIONAL},

};

Introduce four input errors to identify specific input failures
(in addition to generic argument value errors like EINVAL, ERANGE,
EBADF, and E2BIG).

ZFS_ERR_IOC_CMD_UNAVAIL the ioctl number is not supported by kernel
ZFS_ERR_IOC_ARG_UNAVAIL an input argument is not supported by kernel
ZFS_ERR_IOC_ARG_REQUIRED a required input argument is missing
ZFS_ERR_IOC_ARG_BADTYPE an input argument has an invalid type

Reviewed by: allanjude
Obtained from: OpenZFS
Sponsored by: Netflix, Klara Inc.
Differential Revision: https://reviews.freebsd.org/D25393