Cygwin: pty: Introduce disable_pcon in environment CYGWIN.

- For programs which does not work properly with pseudo console,
  disable_pcon in environment CYGWIN is introduced. If disable_pcon
  is set, pseudo console support is disabled.

winsup/cygwin/environ.cc

@@ -120,6 +120,7 @@ static struct parse_thing
   {"reset_com", {&reset_com}, setbool, NULL, {{false}, {true}}},
   {"wincmdln", {&wincmdln}, setbool, NULL, {{false}, {true}}},
   {"winsymlinks", {func: set_winsymlinks}, isfunc, NULL, {{0}, {0}}},
+  {"disable_pcon", {&disable_pcon}, setbool, NULL, {{false}, {true}}},
   {NULL, {0}, setdword, 0, {{0}, {0}}}
 };