Add support for pthread_condattr_t on Unix platforms.

Both pthread_condattr_setclock and pthread_condattr_getclock are guarded
by #if defined(_NETBSD_SOURCE) on NetBSD.

Following types and function have been added:
* pthread_cond_init
* pthread_condattr_t
* pthread_condattr_init
* pthread_condattr_destroy
* pthread_condattr_setclock [0]
* pthread_condattr_getclock [0][1]

[0] Not available on OSX.
[1] May be unavailable on NetBSD, it is a very recent addition.
    Left out from openbsdlike hierarchy for now.

Cast unsigned literals to signed integers.

This makes it explicity that we know the literals are to large and
will result in a negative number. This gets rid of a warning in
1.9.0 and younger versions, as well as an error when using the
constants as discrimintants of #[repr(i32)] enumerations in older
versions.

Linux: add reboot() and constants

This reverts commit ea0a870b.

This reverts commit 43ab51cb.

Add `if_nameindex()` and `if_freenameindex()` from POSIX-2001

Copy them into individual platform files instead. Fixes musl build.

This change is due to an unfortunate interaction between the `s!` macro
and the Android abi. When the struct is defined, but excluded through a
`[cfg(not(target_os = "android"))]` the macro tries to define a
non-existent struct. This commit manually add the functions & struct to
each platform independently.

solaris: add dlsym/dlopen RTLD constants

These constants are taken from OpenIndiana's `dlfcn.h`.  In particular,
`RTLD_DEFAULT` is now needed in `fetch` for std's weak symbol support.

Bump to 0.2.14

Add SIGCHLD and other signals

Added linger struct

Add SIGCHLD, SIGSTOP, SIGTSTP, SIGCONT, SIGTTIN, SIGTTOU. Improve
consistency across oses, by adding SIGEMT to relevant oses.

Add epoll_create1.

Add strdup extern

enhance pathconf constants to cover the most common constants

Add missing consts for Solaris/Illumos

linux: /usr/include/bits/confname.h (taken from redhat, checked against https://github.com/cpc26/abi_linux/blob/master/linux-abi/branches/IBCS3/cxenix/pathconf.c)
android: https://android.googlesource.com/platform/development/+/android-4.3_r1.1/ndk/platforms/android-3/include/pathconf.h
mac: taken from /usr/include/sys/unistd.h

Bump to 0.2.13

unix: Add lchown

Add SEEK_DATA/SEEK_HOLE constants

Relates to https://github.com/nix-rust/nix/pull/377.  These constants
are only available in systems running the Linux kernel.