core: use runuser instead of sudo for switching root->user
core: use runuser instead of sudo for switching root->user

There are problems with using sudo in early system startup
(systemd-logind not running yet, pam_systemd timeouts). Since we don't
need full session here, runuser is good enough (even better: faster).

woju/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,woju/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin