add SIGUSR1 as trigger to print detailed connection information and stats

The SIGUSR1 signal will now cause the OpenConnect CLI to print detailed information about the status of the connection and packet statistics:

$ openconnect -b ...
<connection starts up, goes to background>

$ killall -USR1 openconnect
Configured as A.B.C.D+2612:abcd::f00f/64, with SSL connected and DTLS connected
RX: 415705 packets (340239201 B); TX: 309765 packets (56852637 B)
SSL ciphersuite: (TLS1.2)-(ECDHE-SECP256R1)-(RSA-SHA512)-(AES-256-GCM)
DTLS ciphersuite: (DTLS1.2)-(ECDHE-RSA)-(AES-256-GCM)
Next DTLS rekey in 1427 seconds.

This patch also defers the switch from stderr to syslog until after the tunnel is fully up, so that initial connection information and background PID will be usefully printed to the console. This is a further refinement of 557ac6cf (!117 (merged)).