Fix multiple shellcheck warnings

Fix multiple shellcheck warnings

The latest version of shellcheck reports the following warnings:

check:307:9: warning: Quote arguments to unset so they're not glob expanded. [SC2184]
check:505:10: warning: Quote arguments to unset so they're not glob expanded. [SC2184]
check:506:10: warning: Quote arguments to unset so they're not glob expanded. [SC2184]
check:539:11: warning: Quote arguments to unset so they're not glob expanded. [SC2184]
new:102:19: note: Expansions inside ${..} need to be quoted separately, otherwise they match as patterns. [SC2295]
common/rc:272:37: warning: -ne treats this as an arithmetic expression. Use != to compare as string (or expand explicitly with $((expr))). [SC2309]
tests/block/008:65:10: warning: Quote arguments to unset so they're not glob expanded. [SC2184]
tests/block/008:71:10: warning: Quote arguments to unset so they're not glob expanded. [SC2184]

This patch fixes the above warnings.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>