patch 8.0.1660: the terminal API "drop" command doesn't support options

Problem:    The terminal API "drop" command doesn't support options.
Solution:   Implement the options.
diff --git a/src/ex_cmds.h b/src/ex_cmds.h
index 41c7c80..48b0253 100644
--- a/src/ex_cmds.h
+++ b/src/ex_cmds.h
@@ -1778,7 +1778,7 @@
     int		regname;	/* register name (NUL if none) */
     int		force_bin;	/* 0, FORCE_BIN or FORCE_NOBIN */
     int		read_edit;	/* ++edit argument */
-    int		force_ff;	/* ++ff= argument (index in cmd[]) */
+    int		force_ff;	/* ++ff= argument (first char of argument) */
 #ifdef FEAT_MBYTE
     int		force_enc;	/* ++enc= argument (index in cmd[]) */
     int		bad_char;	/* BAD_KEEP, BAD_DROP or replacement byte */