efivar(3): Fix some typos and improve style

efivar(3): Fix some typos and improve style

- Fix some typos.
- Remove redundant semicolons from the synopsis section.
- Stylize variable names and types with Vt and Va respectively.
- Use a list to present non-implemented functions.
- Sort the order of the sections.
- Add a history section.
- Use Nm when "libefivar" is mentioned.

PR:		232626
Reported by:	Tiwei Bie <btw@mail.ustc.edu.cn>
Reviewed by:	bcr, imp
Approved by:	krion (mentor, implicit), mat (mentor, implicit)
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D17686