Fixes for forkpty and dprintf tests

See merge request redox-os/relibc!494

The former is more portable, even on Linux if for example sudo/doas is used.

Implement ctermid()

See merge request redox-os/relibc!490

Refactor mktime, timegm and fix wcpncpy test

See merge request redox-os/relibc!487

swscanf and vswscanf implementation proposal

See merge request redox-os/relibc!472

Implement dprintf and vdprintf

See merge request redox-os/relibc!489

Implement getpass()

See merge request redox-os/relibc!488

Use u8 internally instead of `c_char`.

- Remove `stdio` functions and use `File` API completely.
- Remove `unsafe` from wrapper function.
- Move static buffer to `getpass()` instead of keeping it in wrapper.

Returns a Result for better error handling.

Also make fields in `termios` public; required for modifying them.

There's an new type of test: `EXPECT_INPUT_BINS`. These require a `.exp`
file to be present along with the `.c` file. The `.exp` file takes the
produced binary as an argument and sends input to the program. This is
useful for testing functions like `getpass()`.

Exclude dirs during build with no cbindgen.toml

See merge request redox-os/relibc!486

Fixes the panic that occurs on builds.

Process/thread distinction improvements

See merge request redox-os/relibc!485

These can occur in the following scenario:

- thread 1 has blocked signal A
- thread 2 has not blocked signal A
- signal A is sent to a process with thread 1 and thread 2
- thread 1 is simultaneously unblocking signal A, and thus jumps to the
  trampoline
- thread 2 is awoken, but thread 1 won the "fetch_andn" to clear the
  signal bit.

If the signal asm is jumped to automatically after each timer interrupt,
this scenario will also be relatively common due to that.