string-table: Move more implementation logic into functions

Let's move some more implementation logic into functions. We keep
the logic that requires the macro in the macro and move the rest into
functions.

While we're at it, let's also make the parameter declarations of
all the string table macros less clausthrophobic.

src/basic/virt.c

@@ -18,6 +18,7 @@
 #include "log.h"
 #include "macro.h"
 #include "namespace-util.h"
+#include "parse-util.h"
 #include "process-util.h"
 #include "stat-util.h"
 #include "string-table.h"