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 The coshell facility as a whole is obsolete and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/INIT/package.sh

@@ -3114,10 +3114,6 @@ cat $INITROOT/$i.sh
 	export SHELL
 	$show SHELL=$SHELL
 	$show export SHELL
-	COSHELL=$SHELL
-	export COSHELL
-	$show COSHELL=$COSHELL
-	$show export COSHELL
 
 	# tame the environment
 
@@ -3495,7 +3491,7 @@ int main(int argc, char** argv) { return argc || argv; }
 					INITPROTO=$PROTOROOT/src/cmd/INIT
 					note proto convert $PACKAGEROOT/src into $PROTOROOT/src
 					if	test -d $PACKAGEROOT/src/cmd/nmake
-					then	dirs="src/cmd/INIT src/lib/libast src/lib/libardir src/lib/libcoshell src/lib/libpp src/cmd/probe src/cmd/cpp src/cmd/nmake"
+					then	dirs="src/cmd/INIT src/lib/libast src/lib/libardir  src/lib/libpp src/cmd/probe src/cmd/cpp src/cmd/nmake"
 					else	dirs="src"
 					fi
 					(
@@ -6006,8 +6002,6 @@ cat $j $k
 		if	test "$KEEP_SHELL" != 1 && executable $OK/ksh
 		then	SHELL=$INSTALLROOT/bin/$OK/ksh
 			export SHELL
-			COSHELL=$SHELL
-			export COSHELL
 		fi
 		case :$PATH: in
 		*:$INSTALLROOT/bin/$OK:*)