patch 8.0.0621: :stag does not respect 'switchbuf'

Problem:    The ":stag" command does not respect 'switchbuf'.
Solution:   Check 'switchbuf' for tag commands that may open a new window.
            (Ingo Karkat, closes #1681)  Define macros for the return values
            of getfile().
diff --git a/src/vim.h b/src/vim.h
index cbe14e5..caf89d4 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -958,6 +958,14 @@
 #define GETF_ALT	0x02	/* jumping to alternate file (not buf num) */
 #define GETF_SWITCH	0x04	/* respect 'switchbuf' settings when jumping */
 
+/* Return values of getfile() */
+#define GETFILE_ERROR	    1	/* normal error */
+#define GETFILE_NOT_WRITTEN 2	/* "not written" error */
+#define GETFILE_SAME_FILE   0	/* success, same file */
+#define GETFILE_OPEN_OTHER -1	/* success, opened another file */
+#define GETFILE_UNUSED	    8
+#define GETFILE_SUCCESS(x)  ((x) <= 0)
+
 /* Values for buflist_new() flags */
 #define BLN_CURBUF	1	/* may re-use curbuf for new buffer */
 #define BLN_LISTED	2	/* put new buffer in buffer list */