Support for Synology [DSM 7]

[gapple]

This builds on #247 to support changes in DSM7

• A new build id: synology-dsm7
• The package name has changed to PlexMediaServer (spaces removed)
• systemctl command is available on DSM7, so conditionals for finding the start command to run needed to be reordered

[gapple]

I made updates to the installer script, but have not tested them

[gapple]

An odd issue I encountered was that Plex appeared to be stopped, re-installed, and started again on each cron run even if a new version was not available. I originally thought the version parsing / comparison was the issue, but instead it appears to have resolved itself 🤷 .

There was an inconsistency though between the version numbers parsed from filenames (e.g PlexMediaServer-1.24.1.4931-1a38e63c6-aarch64_DSM7.spk ➡️ 1.24.1.4931-1a38e63c6 ) and from the installed version returned by synopkg (e.g. 1.24.1.4931-7000), so I made a change to trim the additional value off each string.