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/os/shell.h

@@ -5,6 +5,17 @@
 
 #include "types.h"
 
+// Flags for mch_call_shell() second argument
+typedef enum {
+  kShellOptFilter = 1,  // filtering text
+  kShellOptExpand = 2,  // expanding wildcards
+  kShellOptCooked = 4,  // set term to cooked mode
+  kShellOptDoOut = 8,   // redirecting output
+  kShellOptSilent = 16, // don't print error returned by command
+  kShellOptRead = 32,   // read lines and insert into buffer
+  kShellOptWrite = 64   // write lines from buffer
+} ShellOpts;
+
 char ** shell_build_argv(char_u *cmd, char_u *extra_shell_arg);
 void shell_free_argv(char **argv);