implemented remote POP3 mailbox.
always display error dialog when connection to SMTP server failed.
fixes stall of SMTP when sending via dialup connection.
win32: abort socket polling if not connected.
session.c: remove idle callback on close, and check GSource in the callback (possibly fixes crashes on cancellation).
win32: fixed a bug that the timeout didn't work.
prevented WSAEWOULDBLOCK on sending/receiving in Win32.
win32: increased SO_SNDBUF and SO_RCVBUF (fixes low performance on sending data).
merged from LibSylph branch.
win32: fixed socket.c and session.c.
win32: implemented sock_set_nonblocking_mode().
made workaround for state machine freeze problem in Win32.
session.c: added extra error checking when writing to buffer.
win32: avoid blocking when reading from socket.
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).