Fills some of the gaps in the stdlibrary

A small example of this is:

require 'posix'

mask = Posix::Sigset.new
mask << "INT"
mask << "USR2"

puts Posix.sigprocmask(Posix::SIG_SETMASK, mask)

Posix.execve("/bin/bash", ["/bin/bash"], {"rawr" => "Thing"})

The notable differences are:

• The environment has becomes the child environmemt, instead of being merged into the current environment awkwardly.
• sigprocmask is a thing you can actually call.
• Posix.execve doesn't reset sigprocmask before exec'ing, unless Process#.spawn, Kernel#exec and friends.