core: AutoWhoX, only search nickname by nickname

When running automatic WhoX, explicitly specify "n" to search by
nickname only.  Don't rely on server defaults.  This fixes an issue
with Quassel's autoWHO causing noise in the Status Buffer uncovered
by InspIRCd 3.0's implementation of WhoX that follows the spec.

Fix up incorrect attempt to escape "%" as "%%".  Apparently,
QString::arg() doesn't need to escape this.

Before: WHO <nickname> %%chtsunfra,<unique_number>
After:  WHO <nickname> n%chtsunfra,<unique_number>

See https://github.com/quakenet/snircd/blob/master/doc/readme.who
And https://doc.qt.io/qt-5/qstring.html#arg

Thanks to @genius3000 for reporting and tracking down this issue!

Fixes #1487