pid1: update manager settings on reload too

pid1: update manager settings on reload too

Most complexity of this patch is due to the fact that some manager settings
(basically the watchdog properties) can be set at runtime and in this case the
runtime values must be retained over daemon-reload or daemon-reexec.

For consistency sake, all watchdog properties behaves now the same way, that
is:

  - Values defined by config files can be overridden by writing the new value
    through their respective D-BUS properties. In this case, these values are
    preserved over reload/reexec until the special value '0' or USEC_INFINITY
    is written, which will then restore the last values loaded from the config
    files. If the restored value is '0' or 'USEC_INFINITY', the watchdogs will
    be disabled and the corresponding device will be closed.

  - Reading the properties from a user instance will return the USEC_INFINITY
    value as these properties are only meaningful for PID1.

  - Writing to one of the watchdog properties of a user instance's will be a
    NOP.

Fixes: #15453