tree-wide: drop !! casts to booleans

They are not needed, because anything that is non-zero is converted
to true.

C11:
> 6.3.1.2: When any scalar value is converted to _Bool, the result is 0 if the
> value compares equal to 0; otherwise, the result is 1.

https://stackoverflow.com/questions/31551888/casting-int-to-bool-in-c-c

src/shared/conf-parser.c

@@ -632,7 +632,7 @@ int config_parse_bool(const char* unit,
                 return fatal ? -ENOEXEC : 0;
         }
 
-        *b = !!k;
+        *b = k;
         return 0;
 }