portage.os.waitpid: handle EINTR for bug #525552

Use a new _eintr_func_wrapper class to wrap waitpid calls and handle
EINTR by calling the function as many times as necessary (until it
returns without raising EINTR).

X-Gentoo-Bug: 525552
X-Gentoo-Bug-url: https://bugs.gentoo.org/show_bug.cgi?id=525552
Acked-by: Alexander Berntsen <bernalex@gentoo.org>