PAM: Get username from UID_MIN via libshadowutils

This avoids hardcoding the "nemo" username and picks the first user
from the system's passwd database instead depending on /etc/login.defs

If resolving the username fails via UID_MIN for any reason, fall back
to using "nemo" as the username for setting the password.

[nemo] Username resolved from UID_MIN via libshadowutils