floats: add borders (MS-DOS MODE)

src/nvim/highlight_defs.h

@@ -101,6 +101,7 @@ typedef enum {
   , HLF_MSGSEP      // message separator line
   , HLF_NFLOAT      // Floating window
   , HLF_MSG         // Message area
+  , HLF_BORDER      // Floating window border
   , HLF_COUNT       // MUST be the last one
 } hlf_T;
 
@@ -155,6 +156,7 @@ EXTERN const char *hlf_names[] INIT(= {
   [HLF_MSGSEP] = "MsgSeparator",
   [HLF_NFLOAT] = "NormalFloat",
   [HLF_MSG] = "MsgArea",
+  [HLF_BORDER] = "FloatBorder",
 });