src/: Simplify, using strpbrk(3)

src/: Simplify, using strpbrk(3)

Checking a boolean (actually, a boolean-like pointer) is more readable
than comparing against a length.

This removes the only uses of strcspn(3) in this project.

strpbrk(3) is a simpler call, even though it has a weird name.  It's
just like strchr(3) but searches for several characters.  I'd have named
it strchrs().

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