Build breakage with libc 0.2.154 from git

[Alan Somers]

Nix builds successfully with the most recently released libc: 0.2.153. But the version currently in git contains some backwards-incompatible changes.

Here is a simplified build output, on FreeBSD amd64:

error[E0308]: mismatched types
   --> src/sys/select.rs:113:65
    |
113 |             range: 0..highest.map(|h| h as usize + 1).unwrap_or(FD_SETSIZE),
    |                                                       --------- ^^^^^^^^^^ expected `usize`, found `i32`
    |                                                       |
    |                                                       arguments to this method are incorrect
    |
help: the return type of this call is `i32` due to the type of the argument passed
   --> src/sys/select.rs:113:23
    |
113 |             range: 0..highest.map(|h| h as usize + 1).unwrap_or(FD_SETSIZE),
    |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^----------^
    |                                                                 |
    |                                                                 this argument influences the return type of `unwrap_or`
note: method defined here
   --> /localhome/somers/.rustup/toolchains/nightly-x86_64-unknown-freebsd/lib/rustlib/src/rust/library/core/src/option.rs:953:12
    |
953 |     pub fn unwrap_or(self, default: T) -> T {
    |            ^^^^^^^^^
help: you can convert an `i32` to a `usize` and panic if the converted value doesn't fit
    |
113 |             range: 0..highest.map(|h| h as usize + 1).unwrap_or(FD_SETSIZE.try_into().unwrap()),
    |                                                                           ++++++++++++++++++++

error[E0308]: mismatched types
   --> src/unistd.rs:826:41
    |
826 |     unsafe { libc::execv(path.as_ptr(), args_p.as_ptr()) };
    |              -----------                ^^^^^^^^^^^^^^^ types differ in mutability
    |              |
    |              arguments to this function are incorrect
    |
    = note: expected raw pointer `*const *mut i8`
               found raw pointer `*const *const i8`
note: function defined here
   --> /usr/home/somers/src/rust/libc/src/unix/mod.rs:843:12
    |
843 |     pub fn execv(prog: *const c_char, argv: *const *mut c_char) -> ::c_int;
    |            ^^^^^

See
rust-lang/libc#3597
rust-lang/libc#3356
rust-lang/libc#3466