xinstall: fix dounpriv logic, add tests

Sponsored by: Klara, Inc.
MFC after: 1 week

(cherry picked from commit 36d67475f5497664d33c41c2f6745dcb30b0ec42)

xinstall: make md5 and ripemd160 conditional

Sponsored by: Klara, Inc.

(cherry picked from commit c7a8e8d372b212c97dde6ce2731db27aa0b2201c)

xinstall: use dynamic bufsize as in cat(1) / cp(1).

Sponsored by: Klara, Inc.

(cherry picked from commit 54d8d0fe12a4996427923048ab4261819774fbd4)

xinstall: alphabetize: upper case precedes lower.

Sponsored by: Klara, Inc.

(cherry picked from commit f44e2577120c60cd92f685037bbb63127e0091e4)