Add OC_CMD_DETACH for "reconnectable abort"

OC_CMD_DETACH is similar to OC_CMD_CANCEL, except it does not tell the
server to invalidate the cookie when exiting.  So it is possible to
reconnect to the same session with "openconnect --cookie <foo>" later.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
main: Refactor signal handling

Eliminate the SIGUSR1/SIGUSR2 debug logging behavior and introduce new
behavior for the following signals:

    SIGINT: Disconnect and logoff, run vpnc-script accordingly
    SIGHUP: Disconnect and run vpnc-script (cookie can be re-used)
    SIGUSR2: Reconnect to server immediately as if DPD triggered.
    SIGTERM: Just die. Disconnect without logoff, no vpnc-script

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Change most PRG_TRACE prints to PRG_DEBUG

Use PRG_TRACE for the really noisy (and performance-impacting) log
prints, like packet info and "No work to do" polling.  Use PRG_DEBUG for
other verbose-but-infrequent debug output.  Change the command line
parsing so that "-v" enables PRG_DEBUG and "-vv" enables PRG_TRACE.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>