Thank for the patch, and nice hanging with you at FOSDEM.

Trying to get a handle on the use case for this. Is this so that you
can put the private key and the preshared key in a single file
together? Is there a situation where the shell redirection trick
doesn't cut it? For example:

wg set wg0 private-key <(head -n 1 bothkeys) preshared-key <(tail -n 1 bothkeys)