osdep: limit thread names to 16 characters

It turns out the glibc people are very clever and return an error if the
thread name exceeds the maximum supported kernel length, instead of
truncating the name. So everyone has to hardcode the currently allowed
Linux kernel name length limit, even if it gets extended later.

Also the Lua script filenames could get too long; use the client name
instead.

Another strange thing is that on Linux, unrelated threads "inherit" the
name by the thread they were created. This leads to random thread names,
because there's not necessarily a strong relation between these threads
(e.g. script command leads to filter recreation -> the filter's threads
are tagged with the script's thread name). Unfortunate.