patch 8.1.1769: 'shellslash' is also used for completion

Problem:    'shellslash' is also used for completion.
Solution:   Add the 'completeslash' option. (Yasuhiro Matsumoto, closes #3612)

src/structs.h

@@ -1198,6 +1198,7 @@ typedef struct hashitem_S
 
 // Initial size for a hashtable.  Our items are relatively small and growing
 // is expensive, thus use 16 as a start.  Must be a power of 2.
+// This allows for storing 10 items (2/3 of 16) before a resize is needed.
 #define HT_INIT_SIZE 16
 
 typedef struct hashtable_S
@@ -2395,6 +2396,9 @@ struct file_buffer
 #ifdef FEAT_INS_EXPAND
     char_u	*b_p_cpt;	// 'complete'
 #endif
+#ifdef BACKSLASH_IN_FILENAME
+    char_u	*b_p_csl;	// 'completeslash'
+#endif
 #ifdef FEAT_COMPL_FUNC
     char_u	*b_p_cfu;	// 'completefunc'
     char_u	*b_p_ofu;	// 'omnifunc'