wifi-presence: Add config for process user/group

wifi-presence: Add config for process user/group

On systems using seccomp, the hostapd socket files will be owned by the
'network' user/group ([source][0]). In this case, if wifi-presence is
run as root/root, then it does not have permissions to open the
hostapd socket files. This was discussed in awilliams/wifi-presence#3.

This change allows the process user/group to be specified in
/etc/config/wifi-presence. If no explicit user/group is set, then the
init script will use the owner of the socket files in /var/run/hostapd/
to determine the appropriate process user/group.

[0]: https://github.com/openwrt/openwrt/blob/ec6293febc244d187e71a6e54f44920be679cde4/package/network/services/hostapd/files/wpad.init#L35-L36

Signed-off-by: Adam Williams <pwnfactory@gmail.com>