util-lib: move runlevel_to_target() to shared/unit-file

It if of course related to /proc/cmdline parsing, but is higher-level
functionality built on top of it. It should be in shared/ because it
is something to be used by pid1 and related utilities, not something for
level-level libraries.

src/debug-generator/debug-generator.c

@@ -12,6 +12,7 @@
 #include "special.h"
 #include "string-util.h"
 #include "strv.h"
+#include "unit-file.h"
 #include "unit-name.h"
 
 static const char *arg_dest = NULL;