Set SIGPIPE to SIG_IGN

This solves #783 (closed).

Rationale: When sendmsg fails due to a socket being closed, it will signal SIGPIPE instead of returning EPIPE. See https://pubs.opengroup.org/onlinepubs/009604599/functions/sendmsg.html:

[EPIPE]
    The socket is shut down for writing, or the socket is connection-mode and is no longer connected. In the latter case, and if the socket is of type SOCK_STREAM, the SIGPIPE signal is generated to the calling thread.

The default behavior of a SIGPIPE signal is to just kill the process, with no core dump or error message. (Let me tell you that debugging this was a bit of a ride...) In the case of openconnect, this means that the process just dies, and does not execute the vpnc-script with the "disconnect" argument. (This in turn typically leads to severely messed up routing.)

There are multiple possible ways of handling this, but I agree with this assessment https://stackoverflow.com/a/108192 that setting the SIGPIPE handler to SIG_IGN is the cleanest, safest and most portable.

I have verified that with this patch GnuTLS instead returns an error code, which triggers openconnect to seamlessly reconnect to the VPN service, instead of just dying quietly.