In the editline(3) branch of the sftp(1) event loop, handle SIGINT

In the editline(3) branch of the sftp(1) event loop, handle SIGINT
rather than ignoring it, such that the user can use Ctrl-C to discard
the currently edited command line and get a fresh prompt, just like
in ftp(1), bc(1), and in shells.

It is critical to not use ssl_signal() for this particular case
because that function unconditionally sets SA_RESTART, but here we
need the signal to interrupt the read(2) in the el_gets(3) event loop.

OK dtucker@ deraadt@