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