Fix two small nits in ctl.conf(5).

MFC after:	3 days
Sponsored by:	The FreeBSD Foundation
Fix descriptors leak.

PR:		bin/191002
Reported by:	Ryan Steinmetz
Submitted by:	mjg
Remove the limit on descriptors that can be send in one nvlist.

Submitted by:	Mariusz Zaborski
Use non-recursive algorithm for traversing nvlists. This also removes
the limit on number of nested nvlists.

Submitted by:	Mariusz Zaborski