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 The coshell facility as a whole is obsolete and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

@@ -164,20 +164,7 @@ const struct shtable3 shtab_builtins[] =
 	"",		0, 0 
 };
 
-#if SHOPT_COSHELL
-#  define _JOB_	"[+?Each \ajob\a can be specified as one of the following:]{" \
-        "[+\anumber\a?\anumber\a refers to a process id.]" \
-        "[+-\anumber\a?\anumber\a refers to a process group id.]" \
-        "[+\apool\a.\anum\a?refers to job \anum\a in background pool named \apool\a.]" \
-        "[+\apool\a?refers to all jobs in background pool named \apool\a.]" \
-        "[+%\anumber\a?\anumber\a refer to a job number.]" \
-        "[+%\astring\a?Refers to a job whose name begins with \astring\a.]" \
-        "[+%??\astring\a?Refers to a job whose name contains \astring\a.]" \
-        "[+%+ \bor\b %%?Refers to the current job.]" \
-        "[+%-?Refers to the previous job.]" \
-	"}"
-#else
-#  define _JOB_	"[+?Each \ajob\a can be specified as one of the following:]{" \
+#define _JOB_	"[+?Each \ajob\a can be specified as one of the following:]{" \
         "[+\anumber\a?\anumber\a refers to a process id.]" \
         "[+-\anumber\a?\anumber\a refers to a process group id.]" \
         "[+%\anumber\a?\anumber\a refer to a job number.]" \
@@ -186,7 +173,6 @@ const struct shtable3 shtab_builtins[] =
         "[+%+ \bor\b %%?Refers to the current job.]" \
         "[+%-?Refers to the previous job.]" \
 	"}"
-#endif
 
 
 const char sh_set[] =