Add various strcase* functions and getline

Adds
* `strcasestr`
* `strcasecmp`
* `strncasecmp`
*  `getline`

I *think* they're semi-universal, but shall see what CI pops up...

Retry all curl operations

As the docker pulls from #1148 appear mostly fixed, but we're seeing occasional curl timeouts still (e.g. https://travis-ci.org/rust-lang/libc/jobs/459715534, https://travis-ci.org/rust-lang/libc/jobs/459611627), so this should improve things at least a bit.

Upgrade Travis to using Xenial environment

One part "lets use the latest things", to several parts "might this fix #1148"

Use crates.io keywords and categories

Closes #651 .

Add alias for MAP_ANONYMOUS to Apple targets.

Closes #1060 .

Closes #651 .

Closes #1060 .

No longer needed and this is done via other means in upstream rustc

Prepare for being included via crates.io into std

This commit prepares the `libc` crate to be included directly into the
standard library via crates.io. More details about this can be found on
rust-lang/rust#56092, but the main idea is that this crate now depends
on core/compiler-builtins explicitly (but off-by-default).

The main caveat here is that this activates `no_core` when building as
part of libstd, which means that it needs to explicitly have an `iter`
and `option` module for the expansion of `for` loops to work.

This commit prepares the `libc` crate to be included directly into the
standard library via crates.io. More details about this can be found on
rust-lang/rust#56092, but the main idea is that this crate now depends
on core/compiler-builtins explicitly (but off-by-default).

The main caveat here is that this activates `no_core` when building as
part of libstd, which means that it needs to explicitly have an `iter`
and `option` module for the expansion of `for` loops to work.

Add SGX C types

cc @jethrogb

core::ffi::c_void is available since rustc 1.30

Add missing condition for musl mips64

unix::uclibc::mips has both support for mips and mips64, but it's
currently only imported for mips.

unix::uclibc::mips has both support for mips and mips64, but it's
currently only imported for mips.

Use Reqwest backend for Appveyor, not Hyper which is deprecated

Since https://github.com/rust-lang-nursery/rustup.rs/pull/1222 Appveyor builds have been complaining, so should stop asking for the Hyper backend

Factor out platforms for which libc is empty

This change shouldn't change any functionality. It just separates the platforms for which `libc` is currently empty (only `wasm32-unknown-unknown`), from those for which it isn't. This is a non-functional change.