Additionally, this adds the `SYMLOOP_MAX` constant in `limits.h` to
replace the `MAX_LEVEL` magic number used during symlink resolution.
`fstat` was kept because it is more difficult than the others to do a
drop-in replacement in terms of `fstatat` for since a naive approach
would cause mutual recursion.
I copied the constants and definitions from musl which we can presume is
correct. Relibc is missing some features which affects our sysconf as
well, such as constants in limits.h or a working resources.h.
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()`.
Connor-GH