refactor code

Added common functions autoconf_check_readable and fail.
Added some TODOs for later completion.
Updated messages from upstream plugins.
fix resource leak in if_err_.c
autoconf should exit with 0 when writing no
mpb: add threads.c plugin
mpb: forgot to add threads to listplugins
mpb: stop advertising processes.c

The upstream plugin does way more nowadays. This is no longer a drop-in
replacement.
mnc: avoid out-of-bounds write using strtok
mnc: avoid sprintf based stackoverflow
mnc: fix arbitrary execution via ../ traversal
mnc: avoid execution via shell meta characters

And remove dependency on /bin/sh.
mnc: support parallel builds
Merge remote branch 'helmut/harden-mnc'
Merge remote branch 'helmut/next'
mpb: convert threads arg parsing to plain one