Speed up devel/gdb startup on FreeBSD

On non-Linux systems that do not have fdwalk(), gdb currently iterates
over all possible file descriptors when checking for open files.
Merge an upstream patch that fixes this by adding a FreeBSD
implementation using kinfo_getfile().

Submitted by: Brandon Bergren <git@bdragon.rtk0.net> (initial patch)
Reviewed by: pizzamig (maintainer)
Differential Revision: https://reviews.freebsd.org/D17426