v0.2.6 - --mem-oc by memory type, NVIDIA GDDR6X P2-restore

--mem-oc now does the safe thing for your card automatically, chosen by
memory type, and covers NVIDIA as well as AMD. Opt-in, off by default,
Linux only (the clock control it uses does not exist on Windows), and the clock
is put back when the miner stops.

• NVIDIA GDDR6X (4090 / 4080 / 4070-family, 3090 / 3080 / 3070 Ti): restores
  the P2 memory downclock back to the rated speed - about 2.5%. That is undoing a
  downclock the driver imposes under compute load, not an overclock.
• AMD GDDR6 (RX 7000 / 6000): raises the memory a conservative amount over
  stock via overdrive - a 7900 XT reaches 1325 MHz, about 5%. Needs
  amdgpu.ppfeaturemask=0xffffffff on the kernel command line + a reboot, and
  the miner tells you so if it is off.
• GDDR6 / GDDR7 NVIDIA cards have no P2 downclock, so they are left alone;
  use --mclk-offset N by hand if you know your card.

No dev fee. make test green on both backends.