Fix truncating of files with <>;file combined with <#pattern

The issue with truncating files was caused by out-of-sync streams.
Details and discussion: https://github.com/att/ast/issues/61

src/cmd/ksh93/sh/io.c: sh_iorestore():
- To be safe, sync all streams before restoring file descriptors.

src/cmd/ksh93/tests/io.sh:
- Add two regression tests for truncating files with this
  combination of redirections.
- The second test, which invokes a -c script, is disabled for now
  as this triggers another corner case bug involving the SH_NOFORK
  optimisaton for -c scripts. That fix is for another commit.

(cherry picked from commit 18fb64840365c2ff4608188e5487bd79d08f67d1)

NEWS

@@ -4,6 +4,12 @@ For full details, see the git log at:
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2020-05-21:
+
+- Fix truncating of files with the combined redirections '<>;file' and
+  '<#pattern'. The bug was caused by out-of-sync streams.
+  Details and discussion: https://github.com/att/ast/issues/61
+
 2020-05-20:
 
 - Fix BUG_ISSETLOOP. Expansions like ${var+set} remained static when used