Clean up and improve manual page for fuser(1)

• Mention option's arguments in the list of options (so that now we mention "-N system" instead of just "-N").
• Stylize signals and other constants like O_APPEND with Dv.
• Sort options.
• Change indentation width for readability.
• Fix a couple of typos.
• Sort symbols list.
• Use Sy instead of Cm for symbols. They are not command modifiers.
• Use Ex -std in the EXIT STATUS section for consistency with other manual pages.
• Use Ql instead of Dq Li for inline code examples as Li has recently been deprecated by mdoc.

Reviewed by: bcr
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D25332