Remove unused libcoshell

The coshell(1) command, which is required for libcoshell to be
useful, is not known to be shipped by any distribution. It was
removed by the ksh-community fork and hence also by 93u+m (in
2940b3f5). The coshell facility as a whole is obsolete and
insecure. For a long time now, the statically linked libcoshell
library has been 40+ kilobytes of dead weight in the ksh binary.

Prior discussion (ksh2020): https://github.com/att/ast/issues/619

src/lib/libcoshell/*:
- Removed.

src/cmd/ksh93/*:
- Remove the SHOPT_COSHELL compiler option (which was enabled) and
  a lot of code that was conditional upon #ifdef SHOPT_COSHELL.

- init.c: e_version[]: Removing SHOPT_COSHELL changed the "J"
  feature identifier in ${.sh.version} to a lowercase "j", which
  was conditional upon SHOPT_BGX (background job extensions).
  But src/cmd/ksh93/RELEASE documents (at 08-12-04, on line 1188):
    | +SHOPT_BGX enables background job extensions. Noted by "J" in
    |  the version string when enabled. [...]
  That is the only available documentation. So change that "j" back
  to a "J", leaving the version string unchanged after this commit.

- jobs.c: job_walk(): We need to keep one 'job_waitsafe(SIGCHLD);'
  call that was conditional upon SHOPT_COSHELL; removing it caused
  a regression test failure in tests/sigchld.sh, 'SIGCHLD blocked
  for script at end of pipeline' (which means that until now, a ksh
  compiled without libcoshell had broken SIGCHLD handling.)

bin/package, src/cmd/INIT/package.sh:
- Don't export COSHELL variable.

src/cmd/ksh93/sh/macro.c

@@ -2062,10 +2062,6 @@ static void comsubst(Mac_t *mp,register Shnode_t* t, int type)
 	mp->shp->argaddr = 0;
 	savemac = *mp;
 	mp->shp->st.staklist=0;
-#ifdef SHOPT_COSHELL
-	if(mp->shp->inpool)
-		return;
-#endif /*SHOPT_COSHELL */
 	if(type)
 	{
 		sp = 0;
@@ -2770,11 +2766,7 @@ static char *special(Shell_t *shp,register int c)
 		return(ltos(shp->st.dolc));
 	    case '!':
 		if(shp->bckpid)
-#if SHOPT_COSHELL
-			return(sh_pid2str(shp,shp->bckpid));
-#else
 			return(ltos(shp->bckpid));
-#endif /* SHOPT_COSHELL */
 		break;
 	    case '$':
 		if(nv_isnull(SH_DOLLARNOD))