fix: support s6-overlay 3.2.3.1 user bundle layout

[nerdv2]

Summary

• Use /etc/s6-overlay/user-bundles.d/user for user bundle membership flags when the directory is available.
• Keep /etc/s6-overlay/s6-rc.d/user as a fallback for s6-overlay 3.2.3.0 and older layouts.
• Prevent s6-service from recreating the deprecated s6-rc.d/user directory on current images.

Root cause

s6-overlay 3.2.3.1 moved the user and user2 bundle definitions from s6-rc.d to user-bundles.d. Individual service definitions still belong in s6-rc.d.

The s6-service helper continued writing service membership flags to s6-rc.d/user/contents.d. Creating that legacy user directory makes s6-overlay disable scanning of user-bundles.d, then compilation fails because the recreated directory is not a complete bundle definition:

s6-rc-compile: fatal: unable to read /etc/s6-overlay/s6-rc.d/user/type: No such file or directory

The directory-based selection in this patch supports both layouts without version parsing.

Verification

• Reproduced the startup failure with shinsenter/php:8.3-fpm-nginx@sha256:ee34b4045dea30558c5bf4e53067de1d3c55e7f6b771236b1119124e4aaf01ec.
• Verified POSIX shell and Bash syntax.
• Verified service creation still writes to the legacy bundle with the previous 3.2.3.0 image digest.
• Verified create, disable, and enable operations use user-bundles.d with 3.2.3.1 and do not recreate s6-rc.d/user.
• Successfully compiled the resulting 3.2.3.1 s6-rc database.
• Successfully booted the affected PHP image with the corrected filesystem for both ENABLE_CRONTAB=0 and ENABLE_CRONTAB=1.

Closes #423.
Closes #424.

[shinsenter]

@nerdv2
Thanks for your great contribution. LGTM!
I've approved test builds, and I'll merge this PR once we confirm the -dev images fix the s6-overlay issues.

https://github.com/shinsenter/php/actions/runs/29418305301