w: Trim whitespace and commas from time and uptime

When producing formatted output, trim leading whitespace and trailing
commas from the human-readable time and uptime before emitting them.
The text output remains unchanged.

Approved by: re (cperciva)
PR: 290089
Fixes: 6e6febb54da9 ("w: Fix idle time in json output, add login/idle times to json output")
Reviewed by: marius.h_lden.org
Differential Revision: https://reviews.freebsd.org/D53167

(cherry picked from commit 4d5789532a940144c869d66505e756ce816f8a50)
(cherry picked from commit ca16a115a0fa4da67b153d0b90c9ec08adc9b2fb)