lib/: Treat strpbrk(3)'s return value as a boolean

lib/: Treat strpbrk(3)'s return value as a boolean

with the meaning "a character was found".

strpbrk(3) is just like strchr(3), but searches for multiple characters.
Both functions have a boolean-like return value, which evaluates to true
if a character was found.

A better name for strpbrk(3) would have been strchrs().

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