libsubprocess: ensure bulk-exec output is terminated

[garlick]

Problem: garbage appears in the logs when the bulkexec imp-kill generates output.

Since bulk-exec always uses unbuffered reads for performance, the output buffers returned by flux_subprocess_read() are not guaranteed to be NULL terminated.

Use "%.*s" instead of "%s" in the log format string for:

• the imp kill on_output() handler
• the fallback in case a user doesn't define an on_output() handler

Fixes #6366

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 83.26%. Comparing base (50f8dc2) to head (df77f35).

Files with missing lines	Patch %	Lines
src/common/libsubprocess/bulk-exec.c	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6368      +/-   ##
==========================================
- Coverage   83.28%   83.26%   -0.03%     
==========================================
  Files         524      524              
  Lines       86230    86230              
==========================================
- Hits        71820    71802      -18     
- Misses      14410    14428      +18     

Files with missing lines	Coverage Δ
src/common/libsubprocess/bulk-exec.c	81.55% <0.00%> (ø)

... and 11 files with indirect coverage changes