Fix terminal echo problem when using PID namespace and killing shell process

[cclerget]

Description of the Pull Request (PR):

By killing shell container process when requesting PID namespace, terminal doesn't echo input characters anymore

Steps to reproduce:
First terminal:
$ singularity shell -p docker://busybox
Second terminal:
$ kill -9 </bin/sh pid>
Type something on first terminal

Checkoff for all PRs:

• I have read the Guidelines for Contributing, and this PR conforms to the stated requirements.
• I have tested this PR locally with a make test
• This PR is NOT against the project's master branch
• I have added myself as a contributor to the contributors's file
• This PR is ready for review and/or merge

Attn: @singularityware-admin

[vsoch]

hey @cclerget ping me when 2.4.1 secure bulid is ready - we should update Singularity Hub for the users when it is. Ty!

[cclerget]

Hey @vsoch, I figure out that I need to create another branch and revert merge to exclude SIF stuff that I merged few days ago. I'll let you know once done

[vsoch]

Awesome @cclerget! There is definitely no rush, we want to make sure that the images that are built with Singularity Hub are using the most up-to-date singularity.

[gmkurtzer]

Thank you!