MFC r337812,r337814,r337820,r341068:

Fix several memory leaks (r337812 & r337814).

The libkqueue tests have several places that leak memory by using an
idiom like:

puts(kevent_to_str(kevp));

Rework to save the pointer returned from kevent_to_str() and then
free() it after it has been used.

r337812 also fixed a bug in the netmap kevent code. The inclusion of
that fix was an oversight that I didn't notice until this
MFC. Reference the code review and PR here in the MFC for
completeness.

r337820 & r341068 were white-space only changes as a follow-up to
r337812 & r337814:

After r337820, which "corrected" some spaces-instead-of-tab whitespace
issues in the libkqueue tests, jmg@ pointed out that these files were
originally space-based, not tab-spaced, and so the correction should
have been to get rid of the tabs that had been introduced in previous
changes, not the spaces. This change does that. This is a whitespace
only change; no functional change is intended.

PR: 206053
Differential Revision: https://reviews.freebsd.org/D16531
Sponsored by: Dell EMC Isilon