pkg/osutil: fix for getrlimit on FreeBSD

maxFD did not build on FreeBSD because syscall.Getrlimit returns an
int64 instead of a uint64.

So we convert the result to a uint64.

Credits to @SamWhited on github for the report + fix.

Fixes #925

Change-Id: I15ab682f2f80463279d24ece21a45c23f9a431e6

pkg/osutil/syscall_posix.go

@@ -60,5 +60,14 @@ func maxFD() (uint64, error) {
 		}
 		return 0, fmt.Errorf("ulimit error: %v", err)
 	}
-	return rlim.Cur, nil
+	// On FreeBSD Getrlimit returns an int64, because (among other things) the
+	// maximum value for the Rlimit struct fields, called RLIM_INFINITY used to
+	// be defined as -1.
+	// According to
+	// https://github.com/freebsd/freebsd/blob/master/sys/sys/resource.h , it looks
+	// like it is now defined as
+	// #define	RLIM_INFINITY	((rlim_t)(((__uint64_t)1 << 63) - 1))
+	// which is the maximum positive value of an int64. So casting to an uint64
+	// should be ok.
+	return uint64(rlim.Cur), nil
 }