CMakeLists: conditionally build custom posix module for Lua 5.1

For older Lua versions we need our own custom posix module because
chances are that the system-wide available one is lacking
clock_gettime().
init: try to load own posix module first, then system-wide

If we have our own local module, use that one. If we don't (for Lua
5.2), require the system one to be found.
message: print byte[] and uint8[] as number of bytes

These data types are typically used to transfer large amounts of binary
data, which is irrelevant to print and can print so much (e.g. for
messages containing point clouds) that performance takes a bad hit and
the output becomes cluttered and virtually useless.

To remedy this, just print "N bytes of data" instead.