conf: allow writing uid mappings with euid != 0
conf: allow writing uid mappings with euid != 0

In case unprivileged users run containers via execute() or a start*() there are
valid cases where they may only want to map their own {g,u}id. Let's not block
them from doing so by requiring geteuid() == 0.

Signed-off-by: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@ubuntu.com>

aeris/lxc,caglar10ur/lxc,thmo/lxc,federicobriata/lxc,caglar10ur/lxc,thmo/lxc,terceiro/lxc,hallyn/lxc,LynxChaus/lxc,hallyn/lxc,aeris/lxc,thmo/lxc,terceiro/lxc,hallyn/lxc,caglar10ur/lxc,ss1h2a3tw/lxc,federicobriata/lxc,LynxChaus/lxc,ss1h2a3tw/lxc,ss1h2a3tw/lxc,terceiro/lxc,aeris/lxc,federicobriata/lxc,LynxChaus/lxc