nearly ready for 2.4

[Jim Meyering]

In pre-release testing on Fedora rawhide, I noticed a test failure
because I hadn't installed "bc" there.  The first patch fixes that.
Then, "sudo make check" was consistently failing or mistakenly
skipping one of the last tests: t9041-undetected-in-use-16th-partition.sh.
In the course of debugging that, I spotted a shell syntax error,
which is fixed by the trivial 2/3 patch.

The failing t9041-undetected... test would always succeed
when run in isolation, yet it was supposed to be insulated
from other scsi_debug-using tests by the locking code.
Confirmed that the lock was working, read about scsi_debug
and realized that I can use it to create more than one device,
which might be useful to know for later...

Finally realized that the large number of partitions created in
the t9040-many-partitions.sh test were not all being removed
immediately upon "rmmod scsi_debug".  Instead, they were being
removed *after* completion of that test, and those delayed device
removals were causing the following test to fail.  The fix is simple,
once you discover the root of the problem ;-)

[PATCH 1/3] build: list bc as a build-requirement
[PATCH 2/3] tests: add double quotes around another use of $VERBOSE
[PATCH 3/3] tests: avoid nasty FP-causing race condition