ctl: fix crash when listing in struc format
 - example: kamcmd core.sockets_list
dialog: lock for updating the cseq

- race can happen on processing two requests at the same time within
  same dialog
parser: add internal flag FL_BODY_MULTIPART

This will be set if the body has been converted to multipart/mixed
or a new part has been appended to it. This will trigger the core
to check and fix the boundaries of the body.
core: add check_boundaries() to build_req_buf_from_sip_req().

This checks and fixes the boundaries of a multipart body if the
FL_BODY_MULTIPART flag is set.
textops: functions to convert, append and remove multipart bodies
textops: add new multipart functions documentation
textops: Refresh README