common: support mounts.conf.d/ drop-in configs

[danishprakash]

The base mounts.conf currently has a single entry that's RHEL-specific, and since there's no provision for drop-in support for mounts.conf, non-RHEL distributions need to resort to overrides to skip this mount config. Even though the mount is a no-op in case the path doesn't exist, shipping that config is misleading.

There's already work going on to move RHEL/distro-specific config as drop-in configs and have a distro-agnostic vanilla config shipped as default. This request follows the same idea. There's no rush for this to be a part of 6.0, though.

[Luap99]

I guess one simple thing we should do is to provide an empty mounts.conf as default instead. And have the RHEL one as part of the rpm dir?

Overall I wonder how much of mounts.conf is actually a relict of the past, technically just having a field in containers.conf these days would be much better. And well we already have fields for volume mounts.
That sounds better then extending mounts.conf IMO. Though I think there is use for it in cri-o which does not use containers.conf so they likely want to keep in and then it make sense to have it shared