Added call stack handling to low-level printers.

Low-level printer functions now appropriately set string global
${ZESHY_PRINT_CALL_STACK_ROOT}, to ensure proper newline delimiting.
Added 0-byte file testers to component "path/file/file". Moved 0-byte
file creators to component "path/posix/size" with the other such
functions. Renamed run() to run_command(). A minor suite of minor
fixes (and surely regressions), as well.