nshlib/cmd_cat: Retry if nsh_read() was interrupted by a signal

[JianyuWang0623]

Summary

nshlib/cmd_cat: Retry if nsh_read() was interrupted by a signal.
When read from stdio of child process through pipe, SIGCHLD received if child exits.

Impact

nshlib/cmd_cat.

Testing

1. Selftest

ls | cat | cat | cat | cat

2. Github CI

[nuttxpr]

[Experimental Bot, please feedback here]

No. This PR summary lacks crucial details. While it states what changed, it doesn't explain why retrying on a signal interrupt is necessary for nsh_read(), specifically in the context of pipes and child processes. The impact section is also too brief, simply restating the affected file. It needs to address the impact questions listed in the requirements. Finally, relying solely on GitHub CI for testing is insufficient. Local testing details (host and target) and example logs before/after the change are required.