bin/58768: mount_9p: missing pathconf(2) support

[Christoph Badura <bad%bsd.de@localhost>]

>Number:         58768
>Category:       bin
>Synopsis:       ls: /mnt/.git: Operation not supported
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    bin-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Oct 21 20:25:01 +0000 2024
>Originator:     Christoph Badura <bad%bsd.de@localhost>
>Release:        NetBSD 10.99.11
>Organization:
Bozotic Software Testing Labs
>Environment:
	<The following information is extracted from your kernel. Please>
	<append output of "ldd", "ident" where relevant (multiple lines).>
System: NetBSD nb10-99-11.local 10.99.11 NetBSD 10.99.11 (GENERIC) #0: Tue Oct 1 00:07:37 CEST 2024 bad@inspiral-coalescence-ringdown:/Users/bad/netbsd-gsoc/obj/obj.amd64/sys/arch/amd64/compile/GENERIC amd64
Architecture: x86_64
Machine: amd64
sources from 2024-07-28 with unrelated local changes
>Description:
	I've traced this down to usr.bin/puffs/mount_9p/ninepuffs.c not 
	handling pathconf in PUFFSOP_SET(pops, puffs9p, node, ...);
	<precise description of the problem (multiple lines)>
>How-To-Repeat:
# src from 2024-07-28 with unrelated local changes
$ ls src/build.sh
src/build.sh
$ cat src/sys/arch/amd64/conf/GENERIC.local
ifndef xpci
vio9p*  at virtio?                      # Virtio 9P device
endif
$ ( cd src || exit 1; OBJ=../obj ;  ./build.sh -m amd64 -U -T ${OBJ}/tools \
  -O "${OBJ}/obj.${MACHINE}" -D "${OBJ}/destdir.${MACHINE}" \
  -R "${OBJ}/releasedir.${MACHINE}" release iso-image 
$

$ qemu-system-x86_64  -version
$ qemu-system-x86_64  -M q35 -cpu host -accel hvf -smp 2 -m 512 \
  -cdrom obj/releasedir.amd64/images/NetBSD-10.99.11-amd64-dvd.iso \
  -fsdev local,id=src,security_model=none,path=src,readonly=on \
  -device virtio-9p-pci,fsdev=src,mount_tag=src -nographic
# drop to boot prompt
# consdev com0,115200
# menu
# 1. Boot normally
# exit sysinst
( cd /dev && sh MAKEDEV vio9p0; ls -l vio9p0)
mount -t9p -r -o-cu /dev/vio9p0 /mnt
ls /mnt
ls -l /mnt
# total 187
ls: /mnt/.git: Operation not supported

>Fix:
Implement pathconf support in usr.bin/puffs/mount_9p/ninepuffs.c
	<how to correct or work around the problem, if known (multiple lines)>

>Unformatted:
 	<Please check that the above is correct for the bug being reported,>
 	<and append source date of snapshot, if applicable (one line).>