ash: restore value of imported variable on unexport

Shell variables imported from the environment are marked with a
special flag and backslashes in their values are (by default)
replaced with forward slashes.  Sometimes this may not be what
we want.

Modify the 'export' shell built-in so unexporting a variable of
this type restores its original value from the environment and
removes its special flag.  It can then be re-exported.

Adds 32 bytes.

(GitHub issue #428)

shell/ash.c

@@ -15512,6 +15512,15 @@ exportcmd(int argc UNUSED_PARAM, char **argv)
 						}
 #endif
 						vp->flags = ((vp->flags | flag) & flag_off);
+#if ENABLE_PLATFORM_MINGW32
+						/* Unexporting a variable imported from the
+						 * environment restores its original value and
+						 * removes the VIMPORT flag. */
+						if ((vp->flags & VIMPORT) && (flag_off == ~VEXPORT)) {
+							vp->flags &= ~VIMPORT;
+							p = getenv(name);
+						} else
+#endif
 						continue;
 					}
 				}