Handle unexpectedly large lines for hostlists. (Bug 2333.)

While here rework to avoid extraneous xstrcat calls by using
xstrfmtcat instead of snprintf + xstrcat. Collapse line end into
own string for readability.

No performance or functional change, aside from removing possible
line truncation (which will silence additional Coverity warnings).

Removes a double xfree() in slurm_sprint_reservation_info().