Start twrp userspace fastbootd

[x] I am running an official build of TWRP, downloaded from https://twrp.me/Devices/
[x] I am running the latest version of TWRP
[x] I have read the FAQ (https://twrp.me/FAQ/)
[x] I have searched for my issue and it does not already exist

Device codename: enchilada TWRP version: 3.5.2_9-0

WHAT STEPS WILL REPRODUCE THE PROBLEM?

Since fastboot has moved to userspace, which has also been implemented in twrp least year, I wanted to use the userspace fastboot, since my vendor fastboot does not support certain options.

I have tried several methods to achieve this:

`adb reboot fastboot` from LOS18.1

This boots to twrp, but does not start fastboot. I have checked /tmp/recovery.log and found the following messages related to fastboot:

....
I:Loading page decrypt_users
I:Loading page changeTwrpFolder
I:Switching packages (TWRP)
Boot command: boot-recovery
Got arguments from boot message
Startup Commands:  '--fastboot'
ro.boottime.adbd=3054913539
ro.boottime.init=1900
....

This does indicate that the fastboot option is being passed, but for some reason not processed correctly within twrp.

`adb reboot fastboot` while running twrp

This reboots to system.

`twrp reboot fastboot` from the twrp shell

This reboots to system.

From the twrp GUI

I have tried finding an option to reboot into userspace fastboot from the twrp gui, but the only available reboot options are system, power off, recovery, bootloader, edl.

TeamWin / android_device_oneplus_enchilada