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