kldxref: improve style(9)

Address style issues including some previously raised in D13923.

• Use designated initializers for structs
• Always use bracketed return style
• No initialization in declarations
• Align function prototype names
• Remove old commented code/unused includes

Submitted by: Mitchell Horne <mhorne063@gmail.com>
Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D13943