BUG/MINOR: lua: use strlcpy2() not strncpy() to copy sample keywords

BUG/MINOR: lua: use strlcpy2() not strncpy() to copy sample keywords

The lua initialization code which creates the Lua mapping of all converters
and sample fetch keywords makes use of strncpy(), and as such can take ages
to start with large values of tune.bufsize because it spends its time zeroing
gigabytes of memory for nothing. A test performed with an extreme value of
16 MB takes roughly 4 seconds, so it's possible that some users with huge
1 MB buffers (e.g. for payload analysis) notice a small startup latency.
However this does not affect config checks since the Lua stack is not yet
started. Let's replace this with strlcpy2().

This should be backported to all supported versions.