This is a similar filter to f_metadata, only it works on side data. Since
adding side data from a user provided arbitrary binary string is unsafe,
because current code assumes that a side data of a certain kind has the proper
size, this filter only implements selection and deletion. Also, no value
matching support is implemented yet, because there is no uniform way to specify
a side data textually.
Signed-off-by: Marton Balint <cus@passwd.hu>