[ioctl] add implementation of wait ioctl using semaphores

Squashed merge to clean up conflicts in PR #45's multi-fork history Full commit history:

commit 7ab9f34763df62b80562b86cc57ce193ab25cee3
Author: Mattéo Delabre <1370040+matteodelabre@users.noreply.github.com>
Date:   Thu Feb 18 03:33:52 2021 +0100

    Make the client shim a no-op on reMarkable 1 (#47)

    * Make the client shim a no-op on reMarkable 1

    This PR is intended to make it safe to load the `librm2fb_client`
    library even when running on reMarkable 1. This is so that Toltec can
    ship all the binaries that need to access the framebuffer with a dynamic
    dependency on librm2fb_client, and eliminate the need to set the
    LD_PRELOAD environment variable when starting those binaries.

    In the library constructor, the `/sys/devices/soc0/machine` file is read
    to determine whether we’re running on rM1 or rM2, and set the ON_RM2
    flag accordingly. When this flag is off, the wrapped system calls
    directly forward their arguments to the standard library.

    I also cleaned up some parts of the existing code, in particular:

    - Removed repeated function pointer types, when possible.
    - Moved most file globals to block-level static variables.
    - Removed the main function definition that looked redundant.

    Test plan:

    - Built `librm2fb_client.so.1.0.0` and checked that the exported symbols
      did not change compared to the last version (using
      `arm-oe-linux-gnueabi-objdump -T librm2fb_client.so.1.0.0 | grep "DF .text"`),
      except for the removed `main` symbol:
      * `_ZN6QImageC1EiiNS_6FormatE`
      * `_Z7qputenvPKcRK10QByteArray`
      * `ioctl`
      * `open64`
      * `open`
      * `close`
      * `__libc_start_main`
    - **On rM1,** ran `LD_PRELOAD=./librm2fb_client.so.1.0.0 xochitl`,
      checked that no preloading errors occurred and that no messages from the
      shim appeared. Checked that the display worked correctly and that the
      touchscreen was not inverted.
    - **On rM1,** ran `LD_PRELOAD=./librm2fb_client.so.1.0.0 calculator` and
      performed the same checks as in the previous item.
    - **On rM2,** ran `LD_PRELOAD=./librm2fb_client.so.1.0.0 xochitl` and
      checked that no preloading errors occurred, and that the following
      messages from the shim appeared:

    ```
    OPENED SHARED MEM: /dev/shm/swtfb.01 at 72c2e000, errno: 0
    Update fn address: 0x2c050c
    Create th address: 0x2c2798
    Wait for th address: 0x2c1fd4
    shutdown address: 0x2c273c
    REPLACING THE IMAGE with shared memory
    create threads called
    wait clear func called
    ```

      Also checked that the display worked correctly and that the
      touchscreen was not inverted.
    - **On rM2,** ran `LD_PRELOAD=./librm2fb_client.so.1.0.0 calculator` and
      checked that no preloading errors occurred, that the display worked
      correctly, and that the touchscreen was not inverted.

    * Only set RM2FB_* env vars on rM2

commit 495c051adf622afba726a1176470a9a7f6821d11 Author: Glen Sawyer github@snelg.com Date: Tue Feb 9 20:28:45 2021 -0700

sem_timedwait is based on absolute time, not relative (#46)

* sem_timedwait is based on absolute time, not relative
* Fix tv_nsec wraparound
* Simpler representation of 1,000,000,000

commit 3594d5aeeb758e9524902a9884ee6a536100ab79 Author: okay okay@chalk Date: Mon Feb 8 08:09:49 2021 -0800

[wait] update comments

commit e91fc5abcab082e0c4d0e3c4c4244bbae8558313 Author: okay okay@chalk Date: Sun Feb 7 21:55:24 2021 -0800

[ioctl] use sem_timedwait as outlined in task

commit 8725b8e8136e36e96371c02525a4e15ceffb352b Author: okay okay@chalk Date: Sun Feb 7 21:47:39 2021 -0800

[ioctl] add implementation of wait ioctl using semaphores

ddvk / remarkable2-framebuffer

[ioctl] add implementation of wait ioctl using semaphores #49