bump internal libast version; various minor cleanups

These are minor things I accumulated over the last month or so.

Notable changes:

src/lib/libast/features/api,
src/lib/libast/misc/state.c,
src/lib/libast/comp/conf.tab,
src/cmd/ksh93/include/defs.h:
- Bump internal libast version to 20220101L. We've made a few
  additions to the API, at least pathicase (see 71934570, ca3ec200)
  and astconf_long (see c2ac69b2), so this should have been done
  already. This also updates '/opt/ast/bin/getconf _AST_VERSION'.
- Use AST_VERSION instead of outdated _AST_VERSION.
- In state.c, use AST_VERSION instead of hardcoding the version.

src/cmd/ksh93/sh/xec.c:
- Remove 'restorefd' variable, unused as of 42becab6.
- Remove 'cmdrecurse' function and SH_RUNPROG macro; this was once
  used by a few libcmd commands, but ast-open-archive reveals it's
  unused as of ast 1999-12-25.

src/cmd/ksh93/sh/*.c:
- Where available, use e_dot instead of "." for consistency; it is
  defined as an extern so we might as well use it.

src/cmd/ksh93/tests/*.sh:
- When reporting signal names in fails, include the SIG prefix.
- Fix a broken process hang test in subshell.sh.

src/lib/libast/man/sfdisc.3:
- Removed. The interfaces described here never made it out of AT&T;
  they do not exist in any libast version in ast-open-archive.
  Resolves: https://github.com/ksh93/ksh/issues/426

src/cmd/ksh93/include/defs.h

@@ -29,8 +29,8 @@
 #define defs_h_defined
 
 #include	<ast.h>
-#if !defined(AST_VERSION) || AST_VERSION < 20111111L
-#error libast version 20111111 or later is required
+#if !defined(AST_VERSION) || AST_VERSION < 20220101
+#error libast version 20220101 or later is required
 #endif
 #if !_lib_fork
 #error In 2021, ksh joined the 21st century and started requiring fork(2).