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 The coshell facility as a whole is obsolete and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/include/defs.h

@@ -263,7 +263,6 @@ struct shared
 	Namval_t	*typeinit; \
 	Namfun_t	nvfun; \
 	char		*mathnodes; \
-	void		*coshell; \
 	char		*bltin_dir; \
 	struct Regress_s*regress;
 
@@ -437,11 +436,6 @@ extern int		sh_type(const char*);
 extern void             sh_unscope(Shell_t*);
 extern void		sh_utol(const char*, char*);
 extern int 		sh_whence(char**,int);
-#if SHOPT_COSHELL
-   extern int		sh_coaddfile(Shell_t*,char*);
-   extern int		sh_copipe(Shell_t*, int[], int);
-   extern int		sh_coaccept(Shell_t*,int[],int);
-#endif /* SHOPT_COSHELL */
 #if SHOPT_NAMESPACE
     extern Namval_t	*sh_fsearch(Shell_t*,const char *,int);
 #endif /* SHOPT_NAMESPACE */

src/cmd/ksh93/include/jobs.h

@@ -34,22 +34,6 @@
 #endif /* !SIGINT */
 #include	"FEATURE/options"
 
-#if SHOPT_COSHELL
-#   include	<coshell.h>
-#   define	COPID_BIT	(1L<<30)
-    struct cosh
-    {
-	struct cosh	*next;
-	Coshell_t	*coshell;
-	Cojob_t		*cojob;
-	char		*name;
-	short		id;
-    };
-
-    extern pid_t	sh_copid(struct cosh*);
-    extern char  	*sh_pid2str(Shell_t*,pid_t);
-#endif /* SHOPT_COSHELL */
-
 #undef JOBS
 #if defined(SIGCLD) && !defined(SIGCHLD)
 #   define SIGCHLD	SIGCLD
@@ -76,9 +60,6 @@ struct process
 	struct process *p_nxtjob;	/* next job structure */
 	struct process *p_nxtproc;	/* next process in current job */
 	Shell_t		*p_shp;		/* shell that posted the job */
-#if SHOPT_COSHELL
-	Cojob_t		*p_cojob;	/* coshell job */
-#endif /* SHOPT_COSHELL */
 	int		*p_exitval;	/* place to store the exitval */
 	pid_t		p_pid;		/* process id */
 	pid_t		p_pgrp;		/* process group */
@@ -120,9 +101,6 @@ struct jobs
 	char		waitall;	/* wait for all jobs in pipe */
 	char		toclear;	/* job table needs clearing */
 	unsigned char	*freejobs;	/* free jobs numbers */
-#if SHOPT_COSHELL
-	struct cosh	*colist;	/* coshell job list */
-#endif /* SHOPT_COSHELL */
 };
 
 /* flags for joblist */