procio: use the user-supplied delimiter to split large input

procio: use the user-supplied delimiter to split large input

The `fprocopen` function allows users to specify a delimiter chacter
that is used to split very large input lines into smaller chunks. While
the code checks that the caller did actually supply the delimiter, it is
in fact never used to split the string. Instead, the hardcoded default
character ',' is always used to split the string.

Fix the issue by using `cookie->delim` instead.