lib/, src/: Use consistent style using strchr(3) in conditionals

lib/, src/: Use consistent style using strchr(3) in conditionals

While the return value is a pointer, it can be interpreted as a boolean
value meaning "found".  In general, we use explicit comparisons of
pointers to NULL, but in this specific case, let's use that
interpretation, and make an exception, using an implicit conversion to
boolean.

For negative matches, use
	if (!strchr(...))

For positive matches, use
	if (strchr(...))

For positive matches, when a variable is also set, use
	while (NULL != (p = strchr(...)))

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