plainbox:vendor:extcmd: add glibc-based extcmd

This patch adds a new, non-portable implementation of extcmd based on
glibc and certain linux-specific features.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
plainbox:vendor:pyglibc: fix signalfd's repr() method

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
plainbox:runner: optionally use glibc-based extcmd

This patch makes the JobRunner switch to the new glibc-based job runner
on Linux running Python3.4+

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
"automatic merge of lp:~zkrynicki/checkbox/glibc-extcmd/ by tarmac [r=sylvain-pineau][bug=][author=zkrynicki]"