client: add GetHostUser for u@host and .ssh/config (u-root#280)

Since 2017 cpu used $USER as the user. Now that it supports
ssh, it needs better processing.

Add GetHostUser, which takes a string in [user@]host form,
and returns a host and user name as follows:
if user@host form, return the host and user.

Look up the host in .ssh/config; if found, and
there is a UserName record, return that UserName

Otherwise return host and $USER.

If a host entry is found in .ssh/config, the
HostName from that entry is used.

This means that code that used GetHost should start using
GetHostUser.

Signed-off-by: Ron Minnich <[email protected]>
Co-authored-by: Ron Minnich <[email protected]>

client/fns.go

@@ -213,17 +213,14 @@ func GetHostName(host string) string {
 // can be replaced as needed, but that's for the future.
 func GetHostUser(n string) (host, user string) {
 	if u, h, ok := strings.Cut(n, "@"); ok {
-		host, user = GetHostName(h), u
-		return
+		return GetHostName(h), u
 	}
 	// Perhaps a user name is found in .ssh/config
 	if cp := config.Get(n, "User"); len(cp) != 0 {
-		host, user = GetHostName(n), cp
-		return
+		return GetHostName(n), cp
 	}
 	// Last try: the environment.
-	host, user = GetHostName(n), os.Getenv("USER")
-	return
+	return GetHostName(n), os.Getenv("USER")
 }
 
 // GetPort gets a port. It verifies that the port fits in 16-bit space.