Fix some shell issues by adding quotes and replace backticks with $()

This patch fixes the following issues reported by shellcheck:

• Quote default assignments (SC2223)
• Use $() instead of backticks (SC2006)
• Double quote $@ (SC2068)
• Double quote variables in if-statements and other places (SC2086)

While here, fix a whitespace at one end of line instance in the license
text.

Approved by: manu
Differential Revision: https://reviews.freebsd.org/D40604