menuconfig: add gconfig/xconfig support (#1509)

* change "pmbootstrap kconfig_check" to "pmbootstrap kconfig check"
* change "pmbootstrap menuconfig" to "pmbootstrap kconfig edit [-x|-g]"
  (with legacy alias, because the first syntax was referenced to a lot)
* enable X11 interfaces: -x: xconfig, -g: gconfig
* new function to copy the xauthority file:
  pmb.chroot.other.copy_xauthority()
* remove menufconfig() function from the kernel template and all kernel
  aports ([skip ci] because it would rebuild all kernels and run out of
  time). Alpine has dropped this as well, and it wouldn't work with the
  new code anyway.