Doc: Add some info to README
Fix: add clone3 and a missing DRM_IOCTL syscall

The former is needed for recent versions of some common libs, the latter
is needed for FF.
Fix w3m seccomp filter

On some systems, dynlinking makes w3m need a bunch of extra syscalls.
Fix some potentially undeclared envvars

This isn't a substantial piece of software and is security related, so
it's best to keep this permissive.
BATMAN!!!! (this commit has no parents)