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

pkg/sys/rlimit-memory_bsd.go

@@ -0,0 +1,38 @@
+// +build freebsd dragonfly
+
+/*
+ * 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 sys
+
+import "syscall"
+
+// GetMaxMemoryLimit - returns the maximum size of the process's virtual memory (address space) in bytes.
+func GetMaxMemoryLimit() (curLimit, maxLimit uint64, err error) {
+	var rlimit syscall.Rlimit
+	if err = syscall.Getrlimit(syscall.RLIMIT_DATA, &rlimit); err == nil {
+		curLimit = uint64(rlimit.Cur)
+		maxLimit = uint64(rlimit.Max)
+	}
+
+	return curLimit, maxLimit, err
+}
+
+// SetMaxMemoryLimit - sets the maximum size of the process's virtual memory (address space) in bytes.
+func SetMaxMemoryLimit(curLimit, maxLimit uint64) error {
+	rlimit := syscall.Rlimit{Cur: int64(curLimit), Max: int64(maxLimit)}
+	return syscall.Setrlimit(syscall.RLIMIT_DATA, &rlimit)
+}