gril: merge multi-request from nemomobile/ofono

This change merges multi-request support from nemomobile [1].  It's
based on commit 1b4786a0a0d1f702f87d585259350b319b611c1a by Jarko
Poutiainen <Jarko.Poutiainen@oss.tieto.com>.

[1] https://github.com/nemomobile-packages/ofono
Merge pull request #23 from alfonsosanchezbeato/multi-req

gril: merge multi-request from nemomobile/ofono