Properly terminate pinentry protocol (BYE)

This isn't strictly necessary on unix-like systems since it will exit
when the pipes are closed. Unfortunately Windows isn't so nice and neat
and the program will remain open indefinitely even though its inputs and
outputs have been closed. So ask pinentry to terminate gracefully.

src/enchive.c

@@ -495,6 +495,9 @@ pinentry(FILE *pfi, FILE *pfo, char *buf, size_t len, char *prompt)
         pinentry_decode(buf, len, line + 2);
     else
         fatal("pinentry protocol failure");
+
+    if (fprintf(pfi, "BYE\n") < 0 || fflush(pfi) < 0)
+        fatal("pinentry write() -- %s", strerror(errno));
 }
 
 #if defined(__unix__) || defined(__APPLE__) || defined(__HAIKU__)