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 The coshell facility as a whole is obsolete and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/init.c

@@ -91,14 +91,9 @@ char e_version[]	= "\n@(#)$Id: Version "
 #define ATTRS		1
 			"B"
 #endif
-#if SHOPT_COSHELL
-#define ATTRS		1
-			"J"
-#else
 #if SHOPT_BGX
 #define ATTRS		1
-			"j"
-#endif
+			"J"
 #endif
 #if SHOPT_ACCT
 #define ATTRS		1
@@ -199,9 +194,6 @@ typedef struct _init_
 	Namfun_t	SH_VERSION_init;
 	struct match	SH_MATCH_init;
 	Namfun_t	SH_MATH_init;
-#if SHOPT_COSHELL
-	Namfun_t	SH_JOBPOOL_init;
-#endif /* SHOPT_COSHELL */
 #ifdef _hdr_locale
 	Namfun_t	LC_TYPE_init;
 	Namfun_t	LC_NUM_init;
@@ -1028,10 +1020,6 @@ static char *setdisc_any(Namval_t *np, const char *event, Namval_t *action, Namf
 
 static const Namdisc_t SH_MATH_disc  = { 0, 0, get_math, 0, setdisc_any, create_math, };
 
-#if SHOPT_COSHELL
-static const Namdisc_t SH_JOBPOOL_disc  = { 0, 0, 0, 0, setdisc_any, 0, };
-#endif /*  SHOPT_COSHELL */
-
 #if SHOPT_NAMESPACE
     static char* get_nspace(Namval_t* np, Namfun_t *fp)
     {
@@ -1792,11 +1780,6 @@ static Init_t *nv_init(Shell_t *shp)
 	ip->SH_MATCH_init.hdr.nofree = 1;
 	ip->SH_MATH_init.disc = &SH_MATH_disc;
 	ip->SH_MATH_init.nofree = 1;
-#if SHOPT_COSHELL
-	ip->SH_JOBPOOL_init.disc = &SH_JOBPOOL_disc;
-	ip->SH_JOBPOOL_init.nofree = 1;
-	nv_stack(SH_JOBPOOL, &ip->SH_JOBPOOL_init);
-#endif /* SHOPT_COSHELL */
 	ip->SH_VERSION_init.disc = &SH_VERSION_disc;
 	ip->SH_VERSION_init.nofree = 1;
 	ip->LINENO_init.disc = &LINENO_disc;