lib/, src/: Use strsep(3) instead of strtok(3)

lib/, src/: Use strsep(3) instead of strtok(3)

strsep(3) is stateless, and so is easier to reason about.

It also has a slight difference: strtok(3) jumps over empty fields,
while strsep(3) respects them as empty fields.  In most of the cases
where we were using strtok(3), it makes more sense to respect empty
fields, and this commit probably silently fixes a few bugs.

In other cases (most notably filesystem paths), contiguous delimiters
("//") should be collapsed, so strtok(3) still makes more sense there.
This commit doesn't replace such strtok(3) calls.

While at this, remove some useless variables used by these calls, and
reduce the scope of others.

Signed-off-by: Alejandro Colomar <alx@kernel.org>