util: Add sss_ prefix to some functions
Add sss_ prefix to del_seuser and set_seuser for consistency
with sss_get_seuser. Also sss_ prefix makes it clear that
these functions come from SSSD.
Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
Resolves:
https://pagure.io/SSSD/sssd/issue/3618