ps(1): Fix formatting of the "command" field for kernel threads.

ps(1): Fix formatting of the "command" field for kernel threads.

When -H is specified, for kernel threads the command is formatted as
"<proc name>/<td name>" and truncated to MAXCOMLEN.  But each of the
proc name and td name may be up to MAXCOMLEN bytes in length.

Also handle the ki_moretdname field to ensure that the full thread name
gets printed.  This is already handled correctly when formatting for
"-o tdname".

Reported by:	freqlabs
Reviewed by:	freqlabs
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D25840