boost: Add optional Boost.MPI support

Boost.MPI is a C++ interface library to the
message passing interface (MPI) standard.
kernel: 3.10.59 -> 3.10.60
kernel: 3.2.63 -> 3.2.64
kernel: Remove 3.17 buildfix
kernel: Remove 3.15
Actually remove linux_3_15
nixos/unifi: Remove old ROOT.war links before relinking
strongswan: bump
vim: improve *.nix plugin (#4662)

- Sync syntax from source.
- Add indentation conventions.
cmus: update from 2.5.0 to 2.6.0

Adds CUE and pulseaudio support
Drop GNOME 3.10
ConTeXt: Update to a current version & include it in texLiveFull.
vim-configurable: update the patch-version
gtk3 stack: go back to gnome-3.12 branches for now
Allow starting a shell on stage 1 failure

When people copy the ISO to USB it can happen that the root device isn't known. Having a shell greatly improves that situation.
libfm: update frm 1.2.2.1 to 1.2.3
pcmanfm: update from 1.2.2 to 1.2.3
sxiv: update revision

Mainly to kick off Hydra again: it thinks the build is broken because we
used a SSH URL for git before.
mpd: update to 0.19.3
goffice_0_8: Fix build