Fix various compiler warnings and minor issues (#362)

List of changes:
- Fixed some -Wuninitialized warnings and removed some unused variables.

- Removed the unused extern for B_login (re: d8eba9d1).

- The libcmd builtins and the vmalloc memfatal function now handle
  memory errors with 'ERROR_SYSTEM|ERROR_PANIC' for consistency with how
  ksh itself handles out of memory errors.

- Added usage of UNREACHABLE() where it was missing from error handling.

- Extend many variables from short to int to prevent overflows (most
  variables involve file descriptors).

- Backported a ksh2020 patch to fix unused value Coverity issues
  (https://github.com/att/ast/pull/740).

- Note in src/cmd/ksh93/README that ksh compiles with Cygwin on
  Windows 10 and Windows 11, albeit with many test failures.

- Add comments to detail some sections of code. Extensive list of
  commits related to this change:
  ca2443b5, 7e7f1372, 2db9953a, 7003aba4, 6f50ff64, b1a41311,
  222515bf, a0dcdeea, 0aa9e03f, 61437b27, 352e68da, 88e8fa67,
  bc8b36fa, 6e515f1d, 017d088c, 035a4cb3, 588a1ff7, 6d63b57d,
  a2f13c19, 794d1c86, ab98ec65, 1026006d

- Removed a lot of dead ifdef code.

- edit/emacs.c: Hide an assignment to avoid a -Wunused warning. (See
  also https://github.com/att/ast/pull/753, which removed the assignment
  because ksh2020 removed the !SHOPT_MULTIBYTE code.)

- sh/nvdisc.c: The sh_newof macro cannot return a null pointer because
  it will instead cause the shell to exit if memory cannot be allocated.
  That makes the if statement here a no-op, so remove it.

- sh/xec.c: Fixed one unused variable warning in sh_funscope().

- sh/xec.c: Remove a fallthrough comment added in commit ed478ab7
  because the TFORK code doesn't fall through (GCC also produces no
  -Wimplicit-fallthrough warning here).

- data/builtins.c: The cd and pwd man pages state that these builtins
  default to -P if PATH_RESOLVE is 'physical', which isn't accurate:
     $ /opt/ast/bin/getconf PATH_RESOLVE
     physical
     $ mkdir /tmp/dir; ln -s /tmp/dir /tmp/sym
     $ cd /tmp/sym
     $ pwd
     /tmp/sym
     $ cd -P /tmp/sym
     $ pwd
     /tmp/dir
  The behavior described by these man pages isn't specified in the ksh
  man page or by POSIX, so to avoid changing these builtin's behavior
  the inaccurate PATH_RESOLVE information has been removed.

- Mamfiles: Preserve multi-line errors by quoting the $x variable.
  This fix was backported from 93v-.
  (See also <a7e9cc82>.)

- sh/subshell.c: Remove set but not used sp->errcontext variable.

src/cmd/ksh93/sh/init.c

@@ -1063,16 +1063,6 @@ 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_NAMESPACE
-    static char* get_nspace(Namval_t* np, Namfun_t *fp)
-    {
-	if(sh.namespace)
-		return(nv_name(sh.namespace));
-	return((char*)np->nvalue.cp);
-    }
-    static const Namdisc_t NSPACE_disc	= {  0, 0, get_nspace };
-#endif /* SHOPT_NAMESPACE */
-
 #ifdef _hdr_locale
     static const Namdisc_t LC_disc	= {  sizeof(Namfun_t), put_lang };
 #endif /* _hdr_locale */
@@ -1236,9 +1226,6 @@ Shell_t *sh_init(register int argc,register char *argv[], Shinit_f userinit)
 		shgd->lim.clk_tck = getconf("CLK_TCK");
 		shgd->lim.arg_max = getconf("ARG_MAX");
 		shgd->lim.child_max = getconf("CHILD_MAX");
-		shgd->lim.ngroups_max = getconf("NGROUPS_MAX");
-		shgd->lim.posix_version = getconf("VERSION");
-		shgd->lim.posix_jobcontrol = getconf("JOB_CONTROL");
 		if(shgd->lim.arg_max <=0)
 			shgd->lim.arg_max = ARG_MAX;
 		if(shgd->lim.child_max <=0)
@@ -1310,7 +1297,7 @@ Shell_t *sh_init(register int argc,register char *argv[], Shinit_f userinit)
 	sh_siginit(shp);
 	stakinstall(NIL(Stak_t*),nomemory);
 	/* set up memory for name-value pairs */
-	shp->init_context =  nv_init(shp);
+	shp->init_context = nv_init(shp);
 	/* initialize shell type */
 	if(argc>0)
 	{
@@ -1883,10 +1870,7 @@ Dt_t *sh_inittree(Shell_t *shp,const struct shtable2 *name_vals)
 		n++;
 	np = (Namval_t*)sh_calloc(n,sizeof(Namval_t));
 	if(!shgd->bltin_nodes)
-	{
 		shgd->bltin_nodes = np;
-		shgd->bltin_nnodes = n;
-	}
 	else if(name_vals==(const struct shtable2*)shtab_builtins)
 	{
 		shgd->bltin_cmds = np;