patch 8.1.2127: the indent.c file is a bit big

Problem:    The indent.c file is a bit big.
Solution:   Move C-indent code a a new cindent.c file.  Move other
            indent-related code to indent.c. (Yegappan Lakshmanan,
            closes #5031)

src/globals.h

@@ -843,6 +843,8 @@ EXTERN int	can_si INIT(= FALSE);
 EXTERN int	can_si_back INIT(= FALSE);
 #endif
 
+EXTERN int	old_indent INIT(= 0);	// for ^^D command in insert mode
+
 EXTERN pos_T	saved_cursor		// w_cursor before formatting text.
 #ifdef DO_INIT
 	= {0, 0, 0}