Mark *data and *userdata params to specifier_printf() as const

It would be very wrong if any of the specfier printf calls modified
any of the objects or data being printed. Let's mark all arguments as const
(primarily to make it easier for the reader to see where modifications cannot
occur).

src/shared/install-printf.h

@@ -3,4 +3,4 @@
 
 #include "install.h"
 
-int install_full_printf(UnitFileInstallInfo *i, const char *format, char **ret);
+int install_full_printf(const UnitFileInstallInfo *i, const char *format, char **ret);