shared/specifier: fix %u/%U/%g/%G when called as unprivileged user

We would resolve those specifiers to the calling user/group. This is mostly OK
when done in the manager, because the manager generally operates as root
in system mode, and a non-root in user mode. It would still be wrong if
called with --test though. But in systemctl, this would be generally wrong,
since we can call 'systemctl --system' as a normal user, either for testing
or even for actual operation with '--root=…'.

When operating in --global mode, %u/%U/%g/%G should return an error.

The information whether we're operating in system mode, user mode, or global
mode is passed as the data pointer to specifier_group_name(), specifier_user_name(),
specifier_group_id(), specifier_user_id(). We can't use userdata, because
it's already used for other things.

src/shared/install-printf.h

@@ -4,4 +4,9 @@
 #include "install.h"
 #include "unit-name.h"
 
-int install_name_printf(const UnitFileInstallInfo *i, const char *format, const char *root, char **ret);
+int install_name_printf(
+                UnitFileScope scope,
+                const UnitFileInstallInfo *i,
+                const char *format,
+                const char *root,
+                char **ret);