sysctl: accept slash-separated sysctl names

[ijajmulani]

What this PR does / why we need it

Podman rejects slash-separated sysctl keys with:
sysctl 'net/ipv6/conf/bond1.340/autoconf' is not allowed

[root@mymachine ~]# docker run --rm -ti --name c1 --network bond1.340   --sysctl=net/ipv6/conf/bond1.340/autoconf=0   quay.io/libpod/busybox
Error: sysctl 'net/ipv6/conf/bond1.340/autoconf' is not allowed

pkg/sysctl.Validate() only accepts keys whose normalized form matches prefixes like net. or fs.mqueue.. Slash-first sysctl names are valid per sysctl.d(5) but fail the prefix check. This is a problem for network interfaces whose names contain dots (e.g. VLAN interfaces like bond1.340). For those, slash-first notation is required to set per-interface sysctls such as
net/ipv6/conf/bond1.340/autoconf.

Which issue(s) this PR fixes

N/A (no existing issue filed)

Special notes for reviewers

• Normalization follows the same rules as
  opencontainers/runc#3257.
• Validation uses the normalized key; the original user-provided key is
  preserved in the returned map passed to the runtime.
• Runtime path handling is separate (e.g. crun); this PR only fixes
  Podman's allowlist validation in containers/common.

User-visible impact

Users can pass slash-separated sysctls to Podman, e.g.:

podman run --sysctl net/ipv6/conf/bond1.340/autoconf=0 

Dot-first escaped form (net.ipv6.conf.bond1/340.autoconf=0) was already accepted by prefix check; slash-first form was not.

How this was tested
go test ./pkg/sysctl/
Unit tests for convertSysctlVariableToDotsSeparator() and slash-separated VLAN interface keys
Manual: podman run --sysctl net/ipv6/conf/bond1.340/autoconf=0 with macvlan/ipvlan on a VLAN parent interface

[github-actions]

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