cap_fileargs: add fileargs_lstat service

Add fileargs_lstat function to cap_fileargs casper service to be able to
lstat files while in capability mode. It can only lstat files given in
fileargs_init.

Submitted by: Bora Özarslan <borako.ozarslan@gmail.com>
Reviewed by: oshogbo, cem (partial)
MFC after: 3 weeks
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D19548