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
     hdparm
     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
     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
     /sys/class/net/<nic>/duplex (half, full)
     /proc/sys/fs/file-nr
         This (read-only) file contains three numbers: the number of allocated
         file handles (i.e., the number of files presently opened); the number
         of free file handles; and the maximum number of file handles (i.e.,
         the same value as /proc/sys/fs/file-max). If the number of allocated
         file handles is close to the maximum, you should consider increasing
         the maximum. Before Linux 2.6, the kernel allocated file handles
         dynamically, but it didn't free them again. Instead the free file
         handles were kept in a list for reallocation; the "free file handles"
         value indicates the size of that list. A large number of free file
         handles indicates that there was a past peak in the usage of open
         file handles. Since Linux 2.6, the kernel does deallocate freed file
         handles, and the "free file handles" value is always zero.

Demo plugins:

     linereader
     snmp6
     snmp
     softirqs
     solar
     vmstat
     weather