dhcpd: Update 4.1-ESV-R6 -> 4.3.0

Additionally add the ability to run as a non-root user / group by including
paranoia patches in the build.
nixos/dhcp: Modernize ddns-update-style
nixos/dhcpd: Add the ability to drop privileges
nixos/dhcpd: Use dhcp user instead of nobody
nixos/dhcpd: Convert to systemd from upstart
nixos/dhcpd: Wait until network interfaces are configured to start
license: Add isc
dhcp: Update meta information and add maintainership
pidgin-latex-sf: bump reversion, url and hash

Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>

@vcunat: fix URL.
merge #3074: dhcp: update to the latest version
licenses: fixup double isc definition
dwb: update meta section
Add eiskaltdcpp, a p2p client for the DC protocol
eiskaltdcpp: set platforms.all, fix description

Close #3051.
gnome3: add services.xserver.desktopManager.gnome3.sessionPath

Used to add GI_TYPELIB_PATH and LD_LIBRARY_PATH for gnome shell extensions
or other libraries and services.

Not a great solution but may be the start for further work. Let's make
some cool packages work for now.
gpaste: new package

Clipboard management system with GNOME3 integration

https://github.com/Keruspe/GPaste
sl: fix license (close #3072)

From https://github.com/fistonbettens/nixpkgs/commit/7b6591b551afc6a17b
but it failed to apply for me, so I hand-added and corrected the change.
ffmpeg: maintenance update