patch 8.1.1913: not easy to compute the space on the command line

Problem:    Not easy to compute the space on the command line.
Solution:   Add v:echospace. (Daniel Hahler, closes #4732)
diff --git a/src/eval.c b/src/eval.c
index 5c5fd2f..9a2642c 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -201,6 +201,7 @@
     {VV_NAME("termblinkresp",	 VAR_STRING), VV_RO},
     {VV_NAME("event",		 VAR_DICT), VV_RO},
     {VV_NAME("versionlong",	 VAR_NUMBER), VV_RO},
+    {VV_NAME("echospace",	 VAR_NUMBER), VV_RO},
 };
 
 /* shorthand */
@@ -389,6 +390,8 @@
     set_vim_var_nr(VV_TYPE_CHANNEL, VAR_TYPE_CHANNEL);
     set_vim_var_nr(VV_TYPE_BLOB,    VAR_TYPE_BLOB);
 
+    set_vim_var_nr(VV_ECHOSPACE,    sc_col - 1);
+
     set_reg_var(0);  /* default for v:register is not 0 but '"' */
 
 #ifdef EBCDIC