ctfdump: Use getprogname()

Also remove no longer used function getpname().

Reviewed by: markj
Approved by: re (delphij)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D38740

(cherry picked from commit 3dd5524264095ed8612c28908e13f80668eff2f9)
(cherry picked from commit 994eb2fb8a901c883f2bd24fa29c6cd513cb4340)

ctf: Remove unused function prototype for getpname()

(cherry picked from commit f8e1aa85fed08d5f689cb36d65a76d191f6500dd)
(cherry picked from commit eba26d0ba37e647572b3ec285352f778ef0ec9f0)