Bug #2541: Hang in 100% CPU loop while extacting header details using a delimiter...

Bug #2541: Hang in 100% CPU loop while extacting header details using a delimiter other than comma (external_acl_type, access_log_format, external_refresh_check)

strListGetItem() could get stuck in a 100% loop if called with a delimiter
other than ',' and the parsed string contains ','.

This change makes it properly detect ',' as a delimiter even if called
with another delimiter argument like intended. The reason why ',' is always
a delimiter is because this is the delimiter between merged values of a
multi-valued header, and must always be supported as delimiter even if
Cookie uses ';' instead.