Improve and document fast filescan loops (SHOPT_FILESCAN)

From README:

FILESCAN on  Experimental option that allows fast reading of files
             using while < file;do ...; done and allowing fields in
             each line to be accessed as positional parameters.

As SHOPT_FILESCAN has been enabled by default since ksh 93l
2001-06-01, the filescan loop is now documented in the manual page
and the compile-time option is no longer considered experimental.

We must disable this at runtime if --posix is active because it
breaks a portable use case: POSIXly, 'while <file; do stuff; done'
repeatedly excutes 'stuff' while 'file' can successfully be opened
for reading, without actually reading from 'file'.

This also backports a bugfix from the 93v- beta. Reproducer:

$ echo 'one two three' >foo
$ while <foo; do printf '[%s] ' "$@"; echo; done
[one two three]

Expected output:
[one] [two] [three]

The bug is that "$@" acts like "$*", joining all the positional
parameters into one word though it should be generating one word
for each.

src/cmd/ksh93/sh/macro.c: varsub():
- Backport fix for the bug described above. I do not understand the
  opaque macro.c code well enough yet to usefully describe the fix.

src/cmd/ksh93/sh/xec.c: sh_exec():
- Improved sanity check for filescan loop: do not recognise it if
  the simple command includes variable assignments, more than one
  redirection, or an output or append redirection.
- Disable filescan loops if --posix is active.
- Another 93v- fix: handle interrupts (errno==EINTR) when closing
  the input file.

NEWS

@@ -3,6 +3,25 @@ For full details, see the git log at: https://github.com/ksh93/ksh/tree/1.0
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2022-02-15:
+
+- A bug was fixed in fast filescan loops (like 'while <file; do ...; done',
+  enabled by default by the SHOPT_FILESCAN compile-time option) in which
+  "$@" incorrectly acted like "$*" within the do...done block.
+
+- Improved sanity check for filescan loops: they are now only recognised if
+  'while' is followed by a single input redirection without any command
+  name/arguments or variable assignments.
+
+- As SHOPT_FILESCAN has been enabled by default since ksh 93l 2001-06-01,
+  the fast filescan loop is now documented in the manual page and the
+  compile-time option is no longer considered experimental.
+
+- Filescan loops are now disabled at runtime if the --posix option is active
+  as they break a portable use case: POSIXly, 'while <file; do stuff; done'
+  repeatedly excutes 'stuff' while 'file' can successfully be opened for
+  reading, without actually reading from 'file'.
+
 2022-02-12:
 
 - In multibyte locales such as UTF-8, shell input is no longer corrupted when