containers/netns: Fix crtchild() function parameters

The numbers of parameters for crtchild() function which called in
two_children_ns.c and par_chld_ipv6.c is not correct. And it will make
the child process to be a zombie process.

This commit fixes and moves the function definition to the comon.h
header file.

Signed-off-by: shuang.qiu@oracle.com
containers/netns: Fix warnings.

Fix implicit function declaration and old style function definitions.

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>