textops: fix doc. insert_hf append_hf functions id are duplicated.
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(). WIP.

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