devd: raise the maximum allowed client to 50

since the creation of libudev-devd, but also with powerd, recent libusb
changes etc. 10 client is not enough anymore to cover the desktop needs

and end users often ends up with:
sonewconn: pcb 0xfffff8004dd43780 (local:/var/run/devd.seqpacket.pipe)...

raise the maximum allowed client to 50, which should be enough to cover
user requirements.

MFC After: 1 week

(cherry picked from commit 5682eee1efd35fb65751641181ae2a50d86efaab)