Updates the columns in the members table.
Assumes the data has been htmlspecialchar'd.
this function should be used whenever member data needs to be
updated in place of an UPDATE query.
id_member is either an int or an array of ints to be updated.
data is an associative array of the columns to be updated and their respective values.
any string values updated should be quoted and slashed.
the value of any column can be '+' or '-', which mean 'increment'
and decrement, respectively.
if the member's post number is updated, updates their post groups.