Returning TRUE for zero makes no sense. Obviously, zero is not a power
of two.
Also, the function is used to check whether a number has only one bit
(flag) set, so, an alternative name would be "has-one-bit-set", which
also should return FALSE for zero. All callers didn't really care for
the previous meaning "has-at-most-one-bit-set".
This also avoids the issue of checking (x >= 0), which causes
-Wtype-limits warnings for unsigned types. Which was avoided
by doing (x == 0 || x > 0), which caused -Wlogical-op warning,
which then was avoided (x == 0 || (x > 0 && 1)). Just don't.
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