Get native Win32 mode working (at least in a basic state).

Get native Win32 mode working (at least in a basic state).

Makefile.win32: Use mail-smtp backend.  Fix main build rule.
src/ioset-win32.c (ioset_win32_init): Return values appropriately.
  (ioset_win32_loop): Update "now" before dispatching events.
src/ioset.c: Add errno kludges to work properly on Win32.  Use send() and
  recv() rather than write() and read().  Use simple comparisons rather
  than switch statements with only one case.
src/mail-smtp.c: Initial pass at SMTP backend for mail service.  Does not
  yet try to send the mail body, and has not been tested.
src/main-common.c: Move usage(), version(), license() to here ..
src/main.c: .. from here.
src/main-win32.c: Parse command-line options.