vdagentd: parse argv using GLib

All command line options now have long names
as they are required by GLib.

So the supported command line options now are:

    -h, --help

    -d, --debug
    -s, --virtio-serial-port-path
    -S, --vdagentd-socket
    -u, --uinput-device
    -f, --fake-uinput
    -x, --foreground
    -o, --one-session
    -X, --disable-session-integration

Change the types of some global variables that hold the options:
- const char * --> gchar *
- int          --> gboolean

Define DEFAULT_UINPUT_DEVICE as "/dev/uinput",
since there's already DEFAULT_VIRTIO_PORT_PATH, VDAGENTD_SOCKET.

Signed-off-by: Jakub Janků <jjanku@redhat.com>
Acked-by: Victor Toso <victortoso@redhat.com>