Added MAP_FIXED_NOREPLACE and MAP_SHARED_VALIDATE consts.

This addresses #1339 and #1315. I believe the location of the constants is correct as-is, as both flags are linux-specific additions. Let me know if I missed anything.

linux/musl: Add SEEK_HOLE and SEEK_DATA constants

They are defined since Linux 3.1 but not in musl yet.

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

Because we cannot yet bring a more recent musl to the mips and mipsel
architectures, we disable support for these constant until a cascading
update with rust-lang/rust has been done.

This reverts commit 1a1b170a.

This reverts commit 7b171fb1.

They are defined since Linux 3.1 but not in musl yet.

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

Bump version to 0.2.55

This will bring in support for redox as part of the unix target_family, thus allowing https://github.com/rust-lang/rust/pull/60547 to move forward

Update to the latest wasi-sysroot.

 - Rename `wasm32-unknown-wasi` to `wasm32-wasi`.
 - `__wasilibc_rmfileat` was renamed to `__wasilibc_unlinkat`
 - Add bindings for a few more functions and typedefs.

Add futex constants

Adding some Futex constants. To be used in `parking_lot`. See https://github.com/faern/parking_lot/pull/1/files#r283110797

 - Rename `wasm32-unknown-wasi` to `wasm32-wasi`.
 - `__wasilibc_rmfileat` was renamed to `__wasilibc_unlinkat`
 - Add bindings for a few more functions and typedefs.

Improve newlib constants

This is necessary to set socket options on the Nintendo Switch, which uses a socket implementation directly ported from FreeBSD. Reopened from #1334 because I accidentally closed that.

redox: convert to target_family unix

This is the first step to supporting https://github.com/rust-lang/rust/issues/60139.

In order to have a smooth transition, there will need to be a change made in rust at the same time, switching Redox over to the unix target family. See https://github.com/rust-lang/rust/pull/60547

Update CI from ubuntu 16.04 (EOL) to 19.04

Closes #1333 .

See https://github.com/rust-lang/rust/pull/60775 .