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- Use AST_VERSION instead of outdated _AST_VERSION.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/tests/variables.sh

@@ -1052,7 +1052,7 @@ $SHELL -c '
 	exit $((errors + 1))	# a possible erroneous asynchronous fork would cause exit status 0
 ' unset_test "$@"
 (((e = $?) == 1)) || err_exit "Failure in unsetting one or more special variables" \
-	"(exit status $e$( ((e>128)) && print -n / && kill -l "$e"))"
+	"(exit status $e$( ((e>128)) && print -n /SIG && kill -l "$e"))"
 
 # ... unset in virtual subshell inside of nested function
 $SHELL -c '
@@ -1082,7 +1082,7 @@ $SHELL -c '
 	exit $((errors + 1))	# a possible erroneous asynchronous fork would cause exit status 0
 ' unset_subsh_fun_test "$@"
 (((e = $?) == 1)) || err_exit "Unset of special variable(s) in a virtual subshell within a nested function fails" \
-	"(exit status $e$( ((e>128)) && print -n / && kill -l "$e"))"
+	"(exit status $e$( ((e>128)) && print -n /SIG && kill -l "$e"))"
 
 # ... readonly in subshell
 $SHELL -c '
@@ -1106,7 +1106,7 @@ $SHELL -c '
 	exit $((errors + 1))	# a possible erroneous asynchronous fork would cause exit status 0
 ' readonly_test "$@"
 (((e = $?) == 1)) || err_exit "Failure in making one or more special variables readonly in a subshell" \
-	"(exit status $e$( ((e>128)) && print -n / && kill -l "$e"))"
+	"(exit status $e$( ((e>128)) && print -n /SIG && kill -l "$e"))"
 
 # ... subshell leak test
 $SHELL -c '
@@ -1123,7 +1123,7 @@ $SHELL -c '
 	exit $((errors + 1))
 ' subshell_leak_test "$@"
 (((e = $?) == 1)) || err_exit "One or more special variables leak out of a subshell" \
-	"(exit status $e$( ((e>128)) && print -n / && kill -l "$e"))"
+	"(exit status $e$( ((e>128)) && print -n /SIG && kill -l "$e"))"
 
 # ... upper/lowercase test
 $SHELL -c '
@@ -1159,7 +1159,7 @@ $SHELL -c '
 	exit $((errors + 1))
 ' changecase_test "$@"
 (((e = $?) == 1)) || err_exit "typeset -l/-u doesn't work on special variables" \
-	"(exit status $e$( ((e>128)) && print -n / && kill -l "$e"))"
+	"(exit status $e$( ((e>128)) && print -n /SIG && kill -l "$e"))"
 
 # ... unset followed by launching a forked subshell
 $SHELL -c '
@@ -1176,7 +1176,7 @@ $SHELL -c '
 	exit $?
 ' unset_to_fork_test "$@"
 (((e = $?) == 1)) || err_exit "Failure in unsetting one or more special variables followed by launching forked subshell" \
-	"(exit status $e$( ((e>128)) && print -n / && kill -l "$e"))"
+	"(exit status $e$( ((e>128)) && print -n /SIG && kill -l "$e"))"
 
 # ======
 # ${.sh.pid} should be the forked subshell's PID