RUN on allegro never handled output correctly

probably a copy-paste effect

Signed-off-by: Utz-Uwe Haus <lisp@uuhaus.de>
Fix make-shell-string

(external-program::make-shell-string "ls" '(".") '(("PATH" . "")) nil)

yields
 0[11]: (EXTERNAL-PROGRAM::MAKE-SHELL-STRING "ls" (".") (("PATH" . "")) NIL)
 1[11]: (EXTERNAL-PROGRAM::REFORMAT-ENVIRONMENT (("PATH" . "")))
 1[11]: returned ("PATH=''")
 0[11]: returned" #<Printer Error, obj=#x10000000af9: `\"PATH=''\"' is not of the expected type `LIST'>"

because REFORMAT-ENVIRONMENT flattens the list of conses.

ACL and abcl were affected, lispworks is ignoring the environment, others don't
call this codepath

fixes sellout/external-program#16

Signed-off-by: Utz-Uwe Haus <lisp@uuhaus.de>