cleanup output routines in job_info and node_info.c

reduce reliance on fixed-sized buffers for output, helps reduce
warnings from coverity et al.

split up key/value pairs in preparation for JSON output work.
xstrfmtcat exists and is cleaner than snprintf followed by xstrcat.

use a consistent line ending rather than repeat conditional block.

output format should be unchanged, and has been tested to match
on common cases and passes all relevant regression tests.