Implement getpass()

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()`.

src/header/limits/mod.rs

@@ -2,3 +2,4 @@
 
 pub const PATH_MAX: usize = 4096;
 pub const NGROUPS_MAX: usize = 65536;
+pub const PASS_MAX: usize = 128;