patch 9.1.0568: Cannot expand paths from 'cdpath' setting

Problem:  Cannot expand paths from 'cdpath' setting
          (Daniel Hahler)
Solution: Implement 'cdpath' completion, add the new 'dir_in_path'
          completion type (LemonBoy)

fixes #374
closes: #15205

Signed-off-by: LemonBoy <thatlemon@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/usercmd.c

@@ -102,6 +102,7 @@ static struct
     {EXPAND_BREAKPOINT, "breakpoint"},
     {EXPAND_SCRIPTNAMES, "scriptnames"},
 #endif
+    {EXPAND_DIRS_IN_CDPATH, "dir_in_path"},
     {0, NULL}
 };