Implement security configuration

[ddevault]

The commands and config file details can be the same as it was on 0.15, but we should change how we enforce them - only commands run by sway (i.e. via the exec command) should be able to have access to most secure features. We should do this by opening the wayland socket on their behalf and transferring the file descriptors through exec via the WAYLAND_SOCKET environment variable.

[ddevault]

Taking 1.0-beta.1 off of this because the plan sucks. New plan under discussion here:

swaywm/wlr-protocols#27

[emersion]

Prior art: https://github.com/mupuf/libwsm (config files: https://github.com/mupuf/libwsm/tree/master/data/security/wsm/default)

[emersion]

https://gitlab.freedesktop.org/wayland/weston/issues/206