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

lib/libzfsbootenv/libzfsbootenv.abi

@@ -1,6 +1,6 @@
 <abi-corpus version='2.0' architecture='elf-amd-x86_64' soname='libzfsbootenv.so.1'>
   <elf-needed>
-    <dependency name='libzfs.so.4'/>
+    <dependency name='libzfs.so.6'/>
     <dependency name='libnvpair.so.3'/>
     <dependency name='libc.so.6'/>
   </elf-needed>