Add emscripten support

openbsd: map `si_addr` inside `struct siginfo_t`

formally, under OpenBSD, `si_addr` is part of a C union. As we don't use
others members of the union inside `struct siginfo_t`, it should be safe
to directly map the field.

Update the android CI image

It's got a new toolchain which should hopefully support some more modern
definitions.

Add getentropy(2)'s syscall number as a const

I'm not convinced that the NR_* naming scheme is ideal, but it's pretty
good.

I didn't recognize any other syscall numbers in the file, so I started a
new section.

Bump to 0.2.4

Fix compiles on various BSDs

Also document them to prevent future regressions

Add binding for memrchr on Linux

Bump to 0.2.3

Add a load of constants/definitions from nix

This is a blanket import of lots of constants and function from nix-rust

Expose functionality to get peer credentials for a Unix socket on Linux

Add PATH_MAX for FreeBSDlike, defined in /usr/src/sys/sys/syslimits.h

SysV shared memory APIs

Add PATH_MAX

Add KERN_PROC_* constants for OpenBSD

Add some constants for learning about cpus

Add dirent DT_* definitions

Add getxattr()/setxattr() variations