string: remove memscan()

string: remove memscan()

It has no users, so rather than fixing its use of types (first and
foremost c would need to be cast to unsigned char in the comparison
expression) drop it altogether. memchr() ought to be fine for all
purposes.

Take the opportunity and also do some stylistic adjustments to its
surviving sibling function memchr().

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>