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}}}
 };
 

winsup/cygwin/fhandler_tty.cc

@@ -3145,6 +3145,8 @@ is_running_as_service (void)
 bool
 fhandler_pty_master::setup_pseudoconsole ()
 {
+  if (disable_pcon)
+    return false;
   /* If the legacy console mode is enabled, pseudo console seems
      not to work as expected. To determine console mode, registry
      key ForceV2 in HKEY_CURRENT_USER\Console is checked. */

winsup/cygwin/globals.cc

@@ -71,6 +71,7 @@ bool pipe_byte;
 bool reset_com;
 bool wincmdln;
 winsym_t allow_winsymlinks = WSYM_sysfile;
+bool disable_pcon;
 
 bool NO_COPY in_forkee;
 

winsup/doc/cygwinenv.xml

@@ -99,6 +99,12 @@ system call will immediately fail.</para>
 <xref linkend="pathnames-symlinks"></xref>.</para>
 </listitem>
 
+<listitem>
+<para><envar>disable_pcon</envar> - if set, pseudo console support in
+pty will be disabled.  This is for programs which do not work properly
+under pty with pseudo console enabled.  Defaults to not set.</para>
+</listitem>
+
 </itemizedlist>
 
 </sect2>