Cygwin: make option for native inner link handling.

This code has been causing issues with SUBST and mapped network drives,
so add an option (defaulted to on) which can be used to disable it where
needed.  MSYS=nonativeinnerlinks

Author: jeremyd2019

winsup/cygwin/environ.cc

@@ -123,6 +123,7 @@ static struct parse_thing
   {"error_start", {func: error_start_init}, isfunc, NULL, {{0}, {0}}},
   {"export", {&export_settings}, setbool, NULL, {{false}, {true}}},
   {"glob", {func: glob_init}, isfunc, NULL, {{0}, {s: "normal"}}},
+  {"nativeinnerlinks", {&nativeinnerlinks}, setbool, NULL, {{false}, {true}}},
   {"pipe_byte", {&pipe_byte}, setbool, NULL, {{false}, {true}}},
   {"proc_retry", {func: set_proc_retry}, isfunc, NULL, {{0}, {5}}},
   {"reset_com", {&reset_com}, setbool, NULL, {{false}, {true}}},

winsup/cygwin/globals.cc

@@ -74,6 +74,7 @@ bool wincmdln = true;
 winsym_t allow_winsymlinks = WSYM_deepcopy;
 bool disable_pcon;
 bool winjitdebug = false;
+bool nativeinnerlinks = true;
 
 /* Taken from BSD libc:
    This variable is zero until a process has created a pthread.  It is used

winsup/cygwin/path.cc

@@ -3721,7 +3721,8 @@ symlink_info::check (char *path, const suffix_info *suffixes, fs_info &fs,
 	 differ, return the final path as symlink content and set symlen
 	 to a negative value.  This forces path_conv::check to restart
 	 symlink evaluation with the new path. */
-      if ((pc_flags & (PC_SYM_FOLLOW | PC_SYM_NOFOLLOW_REP)) == PC_SYM_FOLLOW)
+      if (nativeinnerlinks
+	  && (pc_flags & (PC_SYM_FOLLOW | PC_SYM_NOFOLLOW_REP)) == PC_SYM_FOLLOW)
 	{
 	  PWCHAR fpbuf = tp.w_get ();
 	  DWORD ret;