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/macro.c

@@ -2161,7 +2161,7 @@ static void comsubst(Mac_t *mp,register Shnode_t* t, int type)
 		{
 			/* special case $(<file) and $(<#file) */
 			register int fd;
-			int r;
+			int r=0;
 			struct checkpt buff;
 			struct ionod *ip=0;
 			sh_pushcontext(mp->shp,&buff,SH_JMPIO);