plugins: verify function prototypes consistency

By including the declarations in the implementations the compiler will
complain about inconsistencies.
plugins: move paths to macros
plugins: favour snprintf over sprintf
plugins: use stdbool.h where it is usefull
plugins: don't use "hz" as a variable name

IBM CC on AIX defines this as a macro.
Merge remote branch 'helmut/cleanup'
node: split inetd out of the node
node: fix whitespace in inetd.c
node: rename munin-inetd to munin-inetd-c