Clear signal handler when child process exits

[polyrabbit]

#5238 propagates signals to child process, but provides no way to clear signal handler and exit goroutine.
The child process is started in a loop, unable to clear signal handler causes false positive warning logs.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.

Project coverage is 53.95%. Comparing base (c2b21f4) to head (6b58778).
Report is 44 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/mount_unix.go	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5245      +/-   ##
==========================================
- Coverage   54.27%   53.95%   -0.33%     
==========================================
  Files         161      162       +1     
  Lines       43776    44359     +583     
==========================================
+ Hits        23758    23932     +174     
- Misses      17336    17729     +393     
- Partials     2682     2698      +16     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.