User networking with an SMB Share while not running as root
Description of problem:
When attempting to write a file to the qemu share, Samba always responds with NT_STATUS_ACCESS_DENIED.

This only happens on the MacOS version of Samba, on Linux it appears to work without issues for now.
Steps to reproduce:
1. Start a VM with a SMB share attached to it
2. Create a test file to upload `touch test-file.txt`
3. Upload the test file `smbclient //10.0.2.4/qemu -c 'put test-file.txt'
Additional information:
QEMU has been using Samba for it's SMB shares for quite some time now.
But in the 4.17.x release a bug has appeared in the MacOS Build of Samba.

I've filed a bug with Samba, and suggested a fix for it.
https://bugzilla.samba.org/show_bug.cgi?id=15215

The origin of the bug lies in the fact that when running SMBD as a non-root user, a function sets `errno` unexpectedly.
But after discussing this with Samba, they concluded that running smbd as an un-privileged user is not a supported use case.

Whilst this is not a QEMU bug per se, it is caused by the fact that QEMU is running smbd in an unsupported manner.

As a side note, on Linux this bug does not appear to exist as of yet.
The Linux version of `unbecome_root` doesn't seem to set `errno`. (tested on a recent ArchLinux install).
But I think this depends on the LibC implementation of setuid/seteuid/setreuid/etc. so I can't say it won't happen in the future, or with a different LibC implementation.