cap_fileargs: Fix a descriptor leak in the service process.

cap_fileargs: Fix a descriptor leak in the service process.

The service handler for fileargs_open() tries to pre-open multiple files
and pass descriptors for each back to the sandboxed process in a single
message.  This is to amortize the cost of round-trips between the two
processes.

The service process adds a "cache" nvlist to the reply to "open",
containing file descriptors for pre-opened files.  However, when adding
that nvlist to the reply, it was making a copy, effectively leaking the
cached descriptors.

While here, fix spelling in a local variable name.

PR:		241226
Reviewed by:	oshogbo
MFC after:	3 days
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D25095