patch 8.1.0542: shiftwidth() does not take 'vartabstop' into account

Problem:    shiftwidth() does not take 'vartabstop' into account.
Solution:   Use the cursor position or a position explicitly passed.
            Also make >> and << work better with 'vartabstop'. (Christian
            Brabandt)
diff --git a/src/version.c b/src/version.c
index 37f4b1f..82b8abe 100644
--- a/src/version.c
+++ b/src/version.c
@@ -793,6 +793,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    542,
+/**/
     541,
 /**/
     540,