Acquire an exclusive lock before writing to any file. Note that in

some cases we never explicitly unlock a file because fclose will do
it implicitly. Closes #172.

include/shared.h

@@ -1832,6 +1832,14 @@ char *logfile_generate_topid ();
 char *logfile_generate_subid ();
 void logfile_append (const char *fmt, ...);
 
+#if F_SETLKW
+void lock_file (FILE *fp);
+void unlock_file (FILE *fp);
+#else
+#define lock_file(dummy) {}
+#define unlock_file(dummy) {}
+#endif
+
 #ifdef _WIN
 void fsync (int fd);
 #endif