Fix some issues with the application of libxo to ls(1)

• Add whitespace trimming to some fields (username, group, size, inode, blocks) to avoid whitespace in JSON strings
• fix -m mode, was invalid JSON (repeated keys), and was missing outer array container
• in -n mode, numeric uids and gids were returned as strings

Approved by: eadler (mentor)
Sponsored by: ScaleEngine Inc.
Differential Revision: https://reviews.freebsd.org/D2854