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/shared/exec-util.c

@@ -505,7 +505,7 @@ ExecCommandFlags exec_command_flags_from_string(const char *s) {
         if (streq(s, "ambient")) /* Compatibility with ambient hack, removed in v258, map to no bits set */
                 return 0;
 
-        idx = string_table_lookup(exec_command_strings, ELEMENTSOF(exec_command_strings), s);
+        idx = string_table_lookup_from_string(exec_command_strings, ELEMENTSOF(exec_command_strings), s);
         if (idx < 0)
                 return _EXEC_COMMAND_FLAGS_INVALID;