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
node/inetd: fix args for execvp()

The first element of the args array of execvp() should be the called program
name, not its 1rst arg.
plugins: add plugins.h in the dist target
Revert "node/inetd: fix args for execvp()"

This reverts commit 7aa72e3dea90efcdea0106f6ec28281420a445e3.

As helmut said :

    I'd leave it as is and be compliant with mini-inetd
    that way you only have to know one syntax and not two
    also stock openbsd inetd also wants argv[0]
add -Wall ... to compiling CFLAGS
drop -O2 from CC_CHECK_CFLAGS_APPEND

autotools adds its own -O2, so it appeared twice
adding extra -W flags
git ignore tags
remove declaration warning
remove 64 bit %ll warning
node: HOST_NAME_MAX is not defined in freebsd