Make linux(4) set the openfiles soft resource limit to 1024 for Linux
applications, which often depend on this being the case. There's a new
sysctl, compat.linux.default_openfiles, to control this behaviour.

Reviewed by: kevans, emaste, bcr (manpages)
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D25177