core/execute: always set $USER and introduce SetLoginEnvironment=

Before this commit, $USER, $HOME, $LOGNAME and $SHELL are only
set when User= is set for the unit. For system service, this
results in different behaviors depending on whether User=root is set.

$USER always makes sense on its own, so let's set it unconditionally.
Ideally $HOME should be set too, but it causes trouble when e.g. getty
passes '-p' to login(1), which then doesn't override $HOME. $LOGNAME and
$SHELL are more like "login environments", and are generally not
suitable for system services. Therefore, a new option SetLoginEnvironment=
is also added to control the latter three variables.

Fixes #23438

Replaces #8227

src/shared/bus-unit-util.c

@@ -1078,7 +1078,8 @@ static int bus_append_execute_property(sd_bus_message *m, const char *field, con
                               "ProtectHostname",
                               "MemoryKSM",
                               "RestrictSUIDSGID",
-                              "RootEphemeral"))
+                              "RootEphemeral",
+                              "SetLoginEnvironment"))
                 return bus_append_parse_boolean(m, field, eq);
 
         if (STR_IN_SET(field, "ReadWriteDirectories",