Fix command history corruption on syntax error (re: e999f6b1)

Analysis: When a syntax error occurs, the shell performs a
longjmp(3) back to exfile() in main.c on line 417:
415|	if(jmpval)
416|	{
417|		Sfio_t *top;
418|		sh_iorestore((void*)shp,0,jmpval);
419|		hist_flush(shp->gd->hist_ptr);
420|		sfsync(shp->outpool);
The first thing it does is restore the file descriptor state
(sh_iorestore), then it flushes the history file (hist_flush), then
it synchronises sfio's logical stream state with the physical
stream state using (sfsync).

However, the fix applied in e999f6b1 caused sh_iorestore() to sync
all sfio streams unconditionally. So this was done before
hist_flush(), which caused unpredictable behaviour, including
temporary and/or permanent history corruption, as this also synched
shp->outpool before hist_flush() had a chance to do its thing.

The fix is to only call sfsync() in sh_iorestore() if we're
actually about to call ftruncate(2), and not otherwise.

Moral of the story: bug fixes should be as specific as possible to
minimise the risk of side effects.

src/cmd/ksh93/sh/io.c: sh_iorestore():
- Only call sfsync() if we're about to truncate a file.

src/cmd/ksh93/tests/pty.sh:
- Add test.

Thanks to Marc Wilson for reporting the bug and to Johnothan King
for finding the commit that introduced it.

Resolves: https://github.com/ksh93/ksh/issues/209
Relevant: https://github.com/att/ast/issues/61

NEWS

@@ -12,6 +12,10 @@ Any uppercase BUG_* names are modernish shell bug IDs.
 - Fixed a bug introduced on 2020-08-19: Ctrl+D would break after an
   interactive shell received SIGWINCH.
 
+- Fixed a bug introduced on 2020-05-21: on an interactive shell, command lines
+  containing a syntax error were not added to the command history file and
+  sometimes corrupted the command history.
+
 2021-03-05:
 
 - Unbalanced quotes and backticks now correctly produce a syntax error