Fix file descriptor and memory leaks in pr(1)

Also, hook NetBSD's pr test into the build, and add three more test cases.

Reported by: Coverity, Valgrind
CID: 271650 271651 271652 271653 271654 271655 271656 271656
CID: 271657 271658 271659 1006939 1006940 1006941 1006942 1009098
Reviewed by: ngie
MFC after: 3 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D9137