backup-mysql: ensure rm path never expands to /*

We should harden borg-backup.service, so at worst the scripts can't delete any (important) files. Perhaps ProtectSystem=strict and ReadWritePaths=<something>.

I'm not 100% sure it will work though:

ReadWritePaths=, ReadOnlyPaths=, InaccessiblePaths=, ExecPaths=, NoExecPaths=

[...]

Note that these settings will disconnect propagation of mounts from the unit's processes to the host. This means that this setting may not be used for services which shall be able to install mount points in the main mount namespace.

mentioned in issue #299

resolved all threads

Merged as 0150c1b1.

closed