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- Use AST_VERSION instead of outdated _AST_VERSION.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/sh/path.c

@@ -319,7 +319,7 @@ static char *dotpaths_lib(Pathcomp_t *pp, char *path)
 	if(last)
 		*last = 0;
 	else
-		path = ".";
+		path = (char*)e_dot;
 	r = stat(path,&statb);
 	if(last)
 		*last = '/';
@@ -1671,7 +1671,7 @@ Pathcomp_t *path_addpath(Pathcomp_t *first, register const char *path,int type)
 		if(*cp==':')
 		{
 			if(type!=PATH_FPATH)
-				first = path_addcomp(first,old,".",type);
+				first = path_addcomp(first,old,e_dot,type);
 			while(*++path == ':');
 		}
 		else