```
emulators/qemu: Enable user networking by default
The current build system doesn't build slirp/user networking
by default, but a LOT of guides on the internet to get started
with qemu uses it.
This modifies the configure script to add a new option to
explicitly enable it.
Reviewed_by: koobs (ports)
Approved by: ???
Differential_Revision: 27778
MFH: <branch|no> <reason>
```