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.

Implement syscall restart

See merge request redox-os/relibc!483

Fix missing modulo in random_bool()

See merge request !482

Refactor rand48 functions with less unsafe

See merge request redox-os/relibc!473