usr.bin/mail: use F_OK instead of 0 in access()

[Gleydson Soares]

hi,

use F_OK macro instead of 0 in access() when cheching by file existence. make 
de code easier to read. no funcional change.

OK ?
Index: cmd2.c
===================================================================
RCS file: /cvs/src/usr.bin/mail/cmd2.c,v
retrieving revision 1.18
diff -u -p -r1.18 cmd2.c
--- cmd2.c      6 Apr 2011 11:36:26 -0000       1.18
+++ cmd2.c      13 Nov 2012 18:54:35 -0000
@@ -166,7 +166,7 @@ save1(char *str, int mark, char *cmd, st
                return(1);
        printf("\"%s\" ", file);
        fflush(stdout);
-       if (access(file, 0) >= 0)
+       if (access(file, F_OK) >= 0)
                disp = "[Appended]";
        else
                disp = "[New file]";