getprogname.3: Add an example
It shows the difference between getprogname() and argv[0].
Reviewed by: yuripv
Approved by: yuripv (src)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D27204
(cherry picked from commit 1ffdcdadf61423dd02ddad82fc4f3f6c39090c8c)