Add detection of pkexec root privilege escalation program (#776437)

Many of the distributions (confirmed for Arch Linux, Debian, Fedora,
OpenSUSE, RedHat/CentOS and Ubuntu but likely many others too) are now
using polkit as the mechanism to authorise privileged actions, including
using it to authorise GParted to run as root in their packages of
GParted.  Therefore make the GParted configure script check for the
pkexec command.

Use pkexec with the --disable-internal-agent option when available,
preventing pkexec using it's own textual authentication agent.  When
running GParted from a desktop icon or menu there is no terminal with
which to interact with the textual authentication agent to it can't be
used.  The desktop's graphical agent must be running.  This matches how
some distributions already use polkit in their own packages of GParted.

Required actions file will be added in the next commit.

    polkit(8)
    https://www.freedesktop.org/software/polkit/docs/latest/polkit.8.html

    pkexec(1)
    https://www.freedesktop.org/software/polkit/docs/latest/pkexec.1.html

Bug 776437 - GParted fails to run as root under Wayland