Move rlimit functions into sys package. (#3824)

This patch addresses below

* go build works for bsd family
* probe total RAM size for bsd family
* make unit testable functions

cmd/server-rlimit_test.go

@@ -0,0 +1,43 @@
+/*
+ * Minio Cloud Storage, (C) 2017 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package cmd
+
+import "testing"
+
+func TestGetMaxCacheSize(t *testing.T) {
+	testCases := []struct {
+		curLimit       uint64
+		totalRAM       uint64
+		expectedResult uint64
+	}{
+		{uint64(0), uint64(0), uint64(0)},
+		{minRAMSize, uint64(0), uint64(0)},
+		{uint64(0), minRAMSize, uint64(0)},
+		{uint64(18446744073709551615), uint64(8115998720), uint64(0)},
+		{uint64(8115998720), uint64(16115998720), uint64(0)},
+		{minRAMSize, minRAMSize, uint64(4294967296)},
+		{minRAMSize, uint64(16115998720), uint64(4294967296)},
+		{uint64(18446744073709551615), uint64(10115998720), uint64(5057999360)},
+	}
+
+	for _, testCase := range testCases {
+		cacheSize := getMaxCacheSize(testCase.curLimit, testCase.totalRAM)
+		if testCase.expectedResult != cacheSize {
+			t.Fatalf("expected: %v, got: %v", testCase.expectedResult, cacheSize)
+		}
+	}
+}