Add an option to limit the log output of builders

This is mostly useful for Hydra to deal with builders that get stuck
in an infinite loop writing data to stdout/stderr.

src/libstore/globals.hh

@@ -153,6 +153,10 @@ struct Settings {
     /* Whether to compress logs. */
     bool compressLog;
 
+    /* Maximum number of bytes a builder can write to stdout/stderr
+       before being killed (0 means no limit). */
+    unsigned long maxLogSize;
+
     /* Whether to cache build failures. */
     bool cacheFailure;