removed debug print.
session_read_data_as_file_cb(): don't write to file until buffer is filled more than the half.
fixed a bug that lost first part of message data on receiving.
reduced memory usage on POP3 session.
reduced memory usage on SMTP session.
changed the license of LibSylph from GPL to LGPL.
fixed signedness warnings.
made some core modules library (libsylph).