Pull request: all: support setgid, setuid on unix

Updates #2763.

Squashed commit of the following:

commit bd2077c6569b53ae341a58aa73de6063d7037e8e
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Fri Jun 4 16:25:17 2021 +0300

    all: move rlimit_nofile, imp docs

commit ba95d4ab7c722bf83300d626a598aface37539ad
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Fri Jun 4 15:12:23 2021 +0300

    all: support setgid, setuid on unix

internal/aghos/user.go

@@ -0,0 +1,11 @@
+package aghos
+
+// SetGroup sets the effective group ID of the calling process.
+func SetGroup(groupName string) (err error) {
+	return setGroup(groupName)
+}
+
+// SetUser sets the effective user ID of the calling process.
+func SetUser(userName string) (err error) {
+	return setUser(userName)
+}