Fix warnings on Windows

fix warnings reported with -Dwarning_level=2 on Windows.

app/src/command.h

@@ -18,6 +18,7 @@
 #   define PRIsizet PRIu32
 # endif
 # define PROCESS_NONE NULL
+# define NO_EXIT_CODE -1u // max value as unsigned
   typedef HANDLE process_t;
   typedef DWORD exit_code_t;
 
@@ -28,6 +29,7 @@
 # define PRIsizet "zu"
 # define PRIexitcode "d"
 # define PROCESS_NONE -1
+# define NO_EXIT_CODE -1
   typedef pid_t process_t;
   typedef int exit_code_t;
 
@@ -35,8 +37,6 @@
 
 #include "config.h"
 
-# define NO_EXIT_CODE -1
-
 enum process_result {
     PROCESS_SUCCESS,
     PROCESS_ERROR_GENERIC,