install: Code cleanup

This is mainy focused on using bool for booleans but also renames some
variables for clarity, adds some explicit comparisons, adds some braces,
with miscellanous style fixes thrown in.

MFC after: 1 week
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D58355

(cherry picked from commit 134ddd372f8213885dd828ab38cec2384fd451db)