package/polkit: add polkitd user and appropriate permissions

package/polkit: add polkitd user and appropriate permissions

The polkit daemon requires a polkitd user with permission to access the
following directories:
  - /etc/polkit-1
  - /usr/share/polkit-1

The /usr/bin/pkexec file must be owned by owned by root with the
permissions 4755 or else the error "pkexec must be setuid root" is
thrown when it's ran.

Signed-off-by: Adam Duskett <Aduskett@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>