Refactor SHELL_* defines into enum typedef

The SHELL_* defines are the bitflags that can be passed to `mch_call_shell`.
The enum is defined in 'os/shell.h', where all shell-related functions will
eventually be defined.

src/ex_cmds2.c

@@ -43,6 +43,7 @@
 #include "undo.h"
 #include "window.h"
 #include "os/os.h"
+#include "os/shell.h"
 
 static void cmd_source(char_u *fname, exarg_T *eap);
 
@@ -3518,7 +3519,7 @@ static char_u **find_locales(void)
   /* Find all available locales by running command "locale -a".  If this
    * doesn't work we won't have completion. */
   char_u *locale_a = get_cmd_output((char_u *)"locale -a",
-      NULL, SHELL_SILENT);
+      NULL, kShellOptSilent);
   if (locale_a == NULL)
     return NULL;
   ga_init2(&locales_ga, sizeof(char_u *), 20);