Run dropbear as for a single non-root user

[HansH111]

Attached some minor adaptions to run a non-root dropbear service and also change the location of the hostkeys used to a user enabled or custom location.

Added 2 cmdline options:
-H default location for hostkeys
this enables to specify in default_options.h the bare hostkey filenames.
in svr-runopts it is determined if you start dropbear as root (location /etc/dropbear) or as non-root (location ~/.ssh)

-U: specify a forced userid
So whatever you type in as username, you will be login with the forced userid... and as bonus the envvar SSH_ORGUSER will be set with the typed in username.
So this way you can still run as root, but always be forced to a certain user and also know which user was requested.

We use this to have dropbear acting as a user service, since we don't have any root rights or /etc access.

[HansH111]

Any idea why that CIFuzz check failed ?

[mkj]

Any idea why that CIFuzz check failed ?

It's not specific to this PR, there's something wrong with fetching the fuzz corpus. I might have to move it to a different server.