ssh: don't use -l option for shells on FreeBSD

Shells on FreeBSD don't support the -l option. This means that when
handling SSH in-process, we can't give the user a login shell, but this
change at least allows connecting at all.

Updates #13338

Signed-off-by: Percy Wegmann <percy@tailscale.com>

ssh/tailssh/incubator.go

@@ -1089,6 +1089,10 @@ func (ia *incubatorArgs) loginArgs(loginCmdPath string) []string {
 
 func shellArgs(isShell bool, cmd string) []string {
 	if isShell {
+		if runtime.GOOS == "freebsd" {
+			// freebsd's shells don't support the "-l" option, so we can't run as a login shell
+			return []string{}
+		}
 		return []string{"-l"}
 	} else {
 		return []string{"-c", cmd}