Add linux_skip_mounts

[trasz]

Summary

Add 'linux_skip_mounts' rc.conf(5) variable, to avoid mounting
Linux-specific filesystems under /compat/linux when 'linux_enable'
is set to YES.

[• ian]

Looks good to me. I've already added linux_skip_mounts=YES to my rc.conf so when I upgrade to 13 I won't forget to do it. :)

[netchild]

Under the assumption that the man page update will be added to https://reviews.freebsd.org/D22277: OK

[tijl]

I'm not 100% sure but I think all the variables in rc.conf are "positive", not "negative", like enable, never disable. So perhaps the variable should be called linux_mounts or linux_mounts_enable or linux_fs_mount or something, and default to YES.

[trasz]

Rename to 'linux_mounts_enable' and flip the meaning.

[emaste]

In D22320#488259, @tijl wrote:

I'm not 100% sure but I think all the variables in rc.conf are "positive", not "negative", like enable, never disable.

Yes, we should avoid negative-sense controls, for sysctls as well.