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

linux: Add splice, tee, vmsplice

Use iOS nightlies instead of a pinned version

fixes the bitrig d_namlen compile error

Define _SC_PAGE_SIZE constants

According to POSIX, sysconf can take _SC_PAGESIZE or _SC_PAGE_SIZE,
which may be the same value.  This constant was define in "apple" as
equal to _SC_PAGESIZE, but not any of the other platforms.

Add definitions for _SC_PAGE_SIZE for all platforms which had
_SC_PAGESIZE defined. On all platforms but Android (Bionic), they are
the same value; on Android _SC_PAGE_SIZE has a different value.