nvlist: Add nvlist_snprintf() and zfs_dbgmsg_nvlist()

Add nvlist_snprintf() to print a nvlist to a buffer. This is basically
the snprintf() version of dump_nvlist(). Along with that, add a
zfs_dbgmsg_nvlist() to print out an nvlist to dbgmsg. This will aid in
debugging.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #17215