PLEDGE_DRM support

When attempting to sandbox arcan or sway using pledge we get an error like

```
socket(AF_NETLINK, SOCK_RAW, NETLINK_KOBJECT_UEVENT) = 31
> /nix/store/c0hkzndf6i162jymxmlirn9l6ypv7p3c-glibc-2.38-23/lib/libc.so.6(__socket+0x14) [0xefbf4]
> /nix/store/k6jc2ilmlr0g9k6qdkr5n96bmdj7xav1-arcan-0.6.2.1-unstable-2023-10-14/bin/arcan(platform_device_init+0x1a0) [0x981d0]
> /nix/store/k6jc2ilmlr0g9k6qdkr5n96bmdj7xav1-arcan-0.6.2.1-unstable-2023-10-14/bin/arcan(main+0x58) [0x25ad8]
> /nix/store/c0hkzndf6i162jymxmlirn9l6ypv7p3c-glibc-2.38-23/lib/libc.so.6(__libc_start_call_main+0x70) [0x2b580]
> /nix/store/c0hkzndf6i162jymxmlirn9l6ypv7p3c-glibc-2.38-23/lib/libc.so.6(__libc_start_main+0x98) [0x2b658]
> /nix/store/k6jc2ilmlr0g9k6qdkr5n96bmdj7xav1-arcan-0.6.2.1-unstable-2023-10-14/bin/arcan(_start+0x30) [0x26df0]
```

In arcan's case, OpenBSD is supported using pledge and the following flags:
if (-1 == pledge("stdio drm sendfd proc rpath wpath", NULL)){
https://github.com/letoram/arcan/blob/5d42727c03177adb3e499dfb3e6a656c88eaacdc/src/platform/posix/psep_open.c#L892C1-L892C63

It would be great to have this version of pledge support "drm" as well to sandbox graphical environments.

https://github.com/openbsd/src/blob/9eae2935d282032ec0783176a11757cdac4c15b1/sys/kern/kern_pledge.c#L1127

Thanks @tkchia for figuring most of this out.

Start by reading the pledge implementation and compare its supported promises with OpenBSD's kern_pledge.c, especially the drm handling referenced in the issue. Use Arcan's psep_open.c example and the reported socket call as the compatibility case; done means graphical environments such as Arcan or sway can use the drm promise while sandboxed.

Written by the indexing model from the issue text.