Fix assignments preceding 'command <special builtin>' (#19)

Ksh was not checking for `command` when running a special builtin,
which caused preceding invocation-local variable assignments to
become global. This is the reproducer from the att/ast#72:

$ foo=BUG command eval ':'
$ echo "$foo"

This no longer prints 'BUG', as ksh now makes sure the command builtin
is not running a special builtin before making invocation-local
variable assignments global.

src/cmd/ksh93/sh/xec.c:
 - Backport the bugfix for BUG_CMDSPASGN from ksh93v- 2013-10-10-alpha.

src/cmd/ksh93/tests/builtins.sh:
 - Add a regression test based on the reproducer in att/ast#72.

TODO

@@ -13,9 +13,6 @@ Fix build system:
 - Reimport the removed nmake. It is necessary for changes in Makefiles
   to take effect. The machine-generated Mamfiles are now used as a fallback,
   but they are not meant to be edited by hand.
-- Reimport the removed pty command (for scripting interactive sessions). This
-  is necessary for the pty.sh regression tests to work, which test ksh as an
-  interactive shell. We want to avoid breaking the interactive shell, too.
 
 ______
 Fix or remove broken or misguided default aliases:
@@ -59,10 +56,6 @@ https://github.com/modernish/modernish/tree/0.16/lib/modernish/cap/
   don't work.
   See also: https://github.com/att/ast/issues/963
 
-- BUG_CMDSPASGN: preceding a "special builtin"[*] with 'command' does not
-  stop preceding invocation-local variable assignments from becoming global.
-  [*] https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_14
-
 - BUG_CMDSPEXIT: preceding a "special builtin"[*] (other than 'eval', 'exec',
   'return' or 'exit') with 'command' does not always stop it from exiting
   the shell if the builtin encounters error.