tftpd:capsicumize tftpd

[hanslu952_gmail.com]

Summary

Enter libcasper service to enter capability mode,and adjust how tftpd interacts with
socket,because it violates capability.
I reimplemented the underlying file operation with cap_fileargs.

Sponsored by: Google, Inc. (GSoC 2024)

Test Plan

Trivial:

$ mkdir /tftproot
$ cd /tftproot
$ vim testfile
$ tftp localhost

tftp> get testfile

Normal:

$ cd /usr/tests/libexec/tftpd
$ kyua test -k Kyuafile
one case testing
$ kyua debug -k Kyuafile functional:testcase

Set up enotcapable to get coredump

sysctl kern.trap_enotcap=1
sysctl kern.corefile= /tmp/coredumps/%N.core
gdb executable corefile

[oshogbo]

libexec/tftpd/tftpd.c

247

Are we in a capability mode?
Why we need casper?

579

Style.

720

Wy style change?

735

Style.

745

Please reformat this whole section.

747

Do we expect function validate_access to enter capability mode? We don't have a better place?