Android: add some of the same LFS functions as Linux

While this appears to be in bionic libc, it's not in any header, so CI
fails to validate it.  Let's just skip that one.

Audit the LFS functions from Linux and add the same for Android if
bionic supports it, according to libc.map from API 23 (Android 6.0).

The standard library is currently limited to API 18, so it may only use
`lseek64`, `pread64`, `pwrite64`, `ftruncate64`, and `readahead`.

Note that `stat64` and `dirent64` are actually identical to the regular
versions, because bionic has always mapped them to the LFS syscalls.
It also sets `O_LARGEFILE` at all times, so `open` matches `open64`.
Still, the explicit LFS names may be useful to match Linux.

Fixes #189.

Roll together a few PRs in the queue

Add `posix_fadvise()` and related constants

Update links in README for transition to rust-lang

PowerPC64 fixes

We no longer need to check for powerpc64le, so remove it.

this fixes the siginfo_t compilation errors

Add a default-on std feature

This adds a `use_std` Cargo feature which disables `#![no_std]` builds of libc,
but is enabled by default. The library will currently continue to link to the
standard library to maintain backwards compatibility with the 0.2 series and
older Rust compilers for now, but this default can possible be changed in the
future.

Rebased again for the latest changes.

The #[cfg()] was looking for 'arget_os' instead of 'target_os'.

Bump to 0.2.7

SIGSTKSZ is the same on OpenBSD, Bitrig, and NetBSD

Rebasing for pull request to be valid.

The #[cfg()] was looking for 'arget_os' instead of 'target_os'.

unix: Add sendfile to platforms that support it

Merging 2 PRs together

The #[cfg()] was looking for 'arget_os' instead of 'target_os'.

Add more RTLD_* constants