Fix conversion of sysctl variable dots and slashes

[mengjiao-liu]

Ref #3256

see

• https://github.com/opencontainers/runc/pull/3254/files#r739936786
• Upgrade preparation to verify sysctl values containing forward slashes by regex kubernetes/kubernetes#102393 (comment)

sysctl conversion function according to the linux sysctl conversion definition.

       Note that either "/" or "."  may be used as separators within
       sysctl variable names. If the first separator is a slash,
       remaining slashes and dots are left intact. If the first
       separator is a dot, dots and slashes are interchanged.
       "kernel.domainname=foo" and "kernel/domainname=foo" are
       equivalent and will cause "foo" to be written to
       /proc/sys/kernel/domainname. Either
       "net.ipv4.conf.enp3s0/200.forwarding" or
       "net/ipv4/conf/enp3s0.200/forwarding" may be used to refer to
       /proc/sys/net/ipv4/conf/enp3s0.200/forwarding. A glob glob(7)
       pattern may be used to write the same value to all matching keys.
       Keys for which an explicit pattern exists will be excluded from
       any glob matching. In addition, a key may be explicitly excluded
       from being set by any matching glob patterns by specifying the
       key name prefixed with a "-" character and not followed by "=",
       see SYNOPSIS.

see following webpages for more details:
https://man7.org/linux/man-pages/man8/sysctl.8.html
https://man7.org/linux/man-pages/man5/sysctl.d.5.html

Related to:

• Upgrade preparation to verify sysctl values containing forward slashes by regex kubernetes/kubernetes#102393
• libct/configs/validate: allow / in sysctl names #3254

[mengjiao-liu]

PTAL @AkihiroSuda @mrunalp

[kolyshkin]

Left some nits.

Can you please separate out the package validate_test -> package validate change to a separate (first) commit?

[kolyshkin]

LGTM except for a single nit, thank you @mengjiao-liu!

Also, this needs to be rebased due to #3261

[kolyshkin]

Ah, and the signed-off-by needs to be added to the commits (see https://github.com/opencontainers/runc/pull/3257/checks?check_run_id=4100751081)

[mengjiao-liu]

Rebased and signed-off-by. Thanks for reminding!

[kolyshkin]

LGTM, thank you!