[Serusers] Moving SDP-related functions into core parser

[Maxim Sobolev]

Folks,

My nathelper module will need to tweak body of the messages, therefore
it will be third module after im and sms that needs body-related
functions. IMO it would probably make a lot of sense to move those
functions into core parser, so that modules will not have to import im
module just to use one or two utility functions.

Please let me know what do you think.

Thanks!

-Maxim