Merge branch 'rs/xopen-reports-open-failures' into maint

Error diagnostics improvement.

* rs/xopen-reports-open-failures:
  use xopen() to handle fatal open(2) failures
  xopen: explicitly report creation failures

contrib/coccinelle/xopen.cocci

@@ -0,0 +1,16 @@
+@@
+identifier fd;
+identifier die_fn =~ "^(die|die_errno)$";
+@@
+(
+  fd =
+- open
++ xopen
+  (...);
+|
+  int fd =
+- open
++ xopen
+  (...);
+)
+- if ( \( fd < 0 \| fd == -1 \) ) { die_fn(...); }