Support for multiple sslh systemd unit configurations

[Matthewacon]

This change presents a new configuration style for sslh with systemd unit files. Where previously, the global sslh configuration would be in /etc/sslh.cfg, the new configuration layout is /etc/sslh/*.cfg.

For each file located in /etc/sslh/ ending in .cfg, the systemd generator will automatically generate the accompanying socket and drop-in configurations. Individual system service files for each configuration are not required and instead are covered by a single service template [email protected] which takes the name of the desired sslh configuration as a target. The new system follows the structure:

• Config File: /etc/sslh/example.cfg
• Systemd Socket: [email protected]
• Systemd Forking Service: [email protected]
• Systemd Select Service: [email protected]
• Systemd Drop-In Configuration:
  {SYSTEMD_RUN_DIR}/{sslh, sslh-select}@example.service.d/example.conf

Note: {SYSTEMD_RUN_DIR} is provided to the systemd-sslh-generator binary when invoked by systemd and is thus portable to multiple distributions.

Generator Changes:

• Added Early systemd generator error logging through /dev/kmsg
• Fixed memory and file descriptor leaks
• Added a global systemd service for sslh-select
• Added systemd-drop-in configurations to prevent the system from running both sslh forking and select units simultaneously, and to associate a configuration's unit with its respective systemd socket configuration

[yrutschle]

should there not be some documentation? I feel like most of the text you wrote above should be in a 'systemd.md' file under doc/. Then again, I know nothing about systemd so most of what you wrote went over my head, maybe it's all convention and doesn't need to be explicited.

[stokito]

Could you also pick the change:
https://salsa.debian.org/debian/sslh/-/merge_requests/2/diffs?commit_id=02703eebaf9c1089447ccaa0828c69ee3a5e773d

Also see related #419

[yrutschle]

I merged the changes in the PR, and I think @stokito your other remarks are all tracked in #419, so I'll close this.
Thanks @Matthewacon for the PR.