kernel: Enable PPP_FILTER by default.

pppd will try to use it to improve efficiency and complain if it's not available
(but is is not mandatory).
Merge remote-tracking branch 'origin/master' into staging.
nixos: increase allocated memory for gnome tests. Closes #4651
bcache-tools: don't use probe-bcache and bcache-register

This allows us not to include any additional binaries for bcache
support in initrd; just an udev rule.
stage-1: add bcache support

This adds necessary udev rule into initrd to support autodetecting
bcache partitions in stage-1. It also fixes 'sh' path for the new
rule to work.
r-RcmdrPlugin.ROC: disable broken build
haskell-graphviz: disable test suite because it runs way too long

The test suite runs for ages, i.e. 1+ hours. That seems a little excessive.
Merge pull request #4742 from abbradar/bcache-stage1

Support bcache volumes in initrd
nixos: iso-image: use syslinux bootloader for USB booting support

This changes the bootloader for iso generation from Grub to
syslinux. In addition this adds USB booting support, so that
"dd" can be used to burn the generated ISO to USB thumbdrives
instead of needing applications like UnetBootin.
nixos: installation: make installation ISOs USB bootable
mysql: use another download url, cdn.mysql.com is slow after 30%
Update SBCL to 1.2.5
sbt: update to version 0.13.6
Merge pull request #4785 from j-keck/master

sbt: update to version 0.13.6
Merge pull request #4772 from abbradar/lbzip2

lbzip2: add package
sage: fix meta information

Correct homagepage, description, and license.
sage: fix download url for version 6.1.1
Add titanous to maintainers list
mythtv: 0.24.2 -> 0.27.4
xbmc: passthrough desktop and icon files