common/fdlimit: Move fdlimit files to separate package (#15850)

* common/fdlimit: Move fdlimit files to separate package

When go-ethereum is used as a library the calling program need to set
the FD limit.

This commit extract fdlimit files to a separate package so it can be
used outside of go-ethereum.

* common/fdlimit: Remove FdLimit from functions signature

* common/fdlimit: Rename fdlimit functions

common/fdlimit/fdlimit_windows.go

@@ -0,0 +1,47 @@
+// Copyright 2016 The go-ethereum Authors
+// This file is part of go-ethereum.
+//
+// go-ethereum is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// go-ethereum is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with go-ethereum. If not, see <http://www.gnu.org/licenses/>.
+
+package fdlimit
+
+import "errors"
+
+// Raise tries to maximize the file descriptor allowance of this process
+// to the maximum hard-limit allowed by the OS.
+func Raise(max uint64) error {
+	// This method is NOP by design:
+	//  * Linux/Darwin counterparts need to manually increase per process limits
+	//  * On Windows Go uses the CreateFile API, which is limited to 16K files, non
+	//    changeable from within a running process
+	// This way we can always "request" raising the limits, which will either have
+	// or not have effect based on the platform we're running on.
+	if max > 16384 {
+		return errors.New("file descriptor limit (16384) reached")
+	}
+	return nil
+}
+
+// Current retrieves the number of file descriptors allowed to be opened by this
+// process.
+func Current() (int, error) {
+	// Please see Raise for the reason why we use hard coded 16K as the limit
+	return 16384, nil
+}
+
+// Maximum retrieves the maximum number of file descriptors this process is
+// allowed to request for itself.
+func Maximum() (int, error) {
+	return Current()
+}