usernsexec: Make err out vebose for unshare error

usernsexec: Make err out vebose for unshare error

Currently if lxc-usernsexec is run on a kernel without user namespaces
enabled the error message is

	unshare: Invalid argument
	read pipe: Success

This error message 'Invalid argument' does not point at the root cause
of the error.  We can help the user out by giving a more detailed error
message and also not using perror() if errno==0.

Improve error message by
 - Printing unshare flags
 - Printing suggested cause of failure (user namespace not enabled)
 - Print error message with fprintf() if errno==0 (EOF)

Signed-off-by: Tobin C. Harding <me@tobin.cc>