runsc: Fix newline for cmd usage display

[copybara-service]

runsc: Fix newline for cmd usage display

Summary

Originally, some of the commands' usage display didn't achieved to show newline, while some of them does, which are more user-friendly, and won't confuse the message if there're more which should be in the nextline.

Fix this issue by returning usage string with a newline.

Tests

Before the change some commands display their usage like the following, for example runsc start

vax-r@vaxr-ASUSPRO-D840MB-M840MB:~/gvisor$ runsc start
start <container id> - start a secure container.vax-r@vaxr-ASUSPRO-D840MB-M840MB:~/gvisor$

Or runsc wait

vax-r@vaxr-ASUSPRO-D840MB-M840MB:~/gvisor$ runsc wait
wait [flags] <container id>  -checkpoint
    	wait for the next checkpoint to complete
  -pid int
    	select a PID in the container's PID namespace to wait on instead of the container's root process (default -1)
  -rootpid int
    	select a PID in the sandbox root PID namespace to wait on instead of the container's root process (default -1)

We can see in the case of runsc wait, the flag -checkpoint is expected to be shown in a way which is aligned with other flags, like

vax-r@vaxr-ASUSPRO-D840MB-M840MB:~/gvisor$ runsc wait
wait [flags] <container id>  
  -checkpoint
    	wait for the next checkpoint to complete
  -pid int
    	select a PID in the container's PID namespace to wait on instead of the container's root process (default -1)
  -rootpid int
    	select a PID in the sandbox root PID namespace to wait on instead of the container's root process (default -1)

After the change , we can see those commands with this issue are being fixed.

vax-r@vaxr-ASUSPRO-D840MB-M840MB:~/gvisor$ runsc start
start <container id> - start a secure container.
vax-r@vaxr-ASUSPRO-D840MB-M840MB:~/gvisor$

vax-r@vaxr-ASUSPRO-D840MB-M840MB:~/gvisor$ runsc wait
wait [flags] <container id>
  -checkpoint
    	wait for the next checkpoint to complete
  -pid int
    	select a PID in the container's PID namespace to wait on instead of the container's root process (default -1)
  -rootpid int
    	select a PID in the sandbox root PID namespace to wait on instead of the container's root process (default -1)

FUTURE_COPYBARA_INTEGRATE_REVIEW=#11324 from vax-r:fix_usage_display 83f9f18