Add SIGUSR1 for update

List of plugins to implement:

     iostat [-x] [-d] [-N] -zk [-p | <devices>...] <interval>
     SMART monitoring
     Battery
     /proc/acpi (mute)
     System tray
     Eyes
     Launchers
     Taskbar (as in rarity)
     Application menu
     UPS
     Blueshift integration
     Backlight control
     Thermal monitoring
     ESSID and link quality for wireless interfaces
     News feed syndication
     Keyboard layout
     EWMH
     Meteor showers
     pty for write/talk messages
     Lunar and solar eclipses
     e-mail
     Identify active and visible windows
     Calendars: holidays, birthdays, name days, events
     Clock plugin with timezone support
     Summer time / standard time announcement (should support double
         summer time and similar)
     natural disaster reports
     IRC
     /proc/dri
     /proc/interrupts
     /proc/net/sockstat
     /proc/net/sockstat6
     /proc/net/wireless

Demo plugins:

     linereader
     snmp6
     snmp
     softirqs
     solar
     vmstat
     weather
     hdparm