core: change KeyringMode= to "shared" by default for non-service units in the system...

core: change KeyringMode= to "shared" by default for non-service units in the system manager (#8172)

Before this change all unit types would default to "private" in the
system service manager and "inherit" to in the user service manager.

With this change this is slightly altered: non-service units of the
system service manager are now run with KeyringMode=shared. This appears
to be the more appropriate choice as isolation is not as desirable for
mount tools, which regularly consume key material. After all mounts are
a shared resource themselves as they appear system-wide hence it makes a
lot of sense to share their key material too.

Fixes: #8159