BUILD/MEDIUM: checks: get rid of sprintf()

BUILD/MEDIUM: checks: get rid of sprintf()

OpenBSD complains about our use of sprintf() here :

src/checks.o(.text+0x44db): In function `process_chk':
src/checks.c:766: warning: sprintf() is often misused, please use snprintf()

This case was not really clean since the introduction of global.node BTW.
Better change the API to support a size argument in the function and enforce
the limit.