MINOR: tools: make parse_line() always terminate the args list

MINOR: tools: make parse_line() always terminate the args list

parse_line() as added in commit c8d167bcf ("MINOR: tools: add a new
configurable line parse, parse_line()") presents an difficult usage
because it's up to the caller to determine the last written argument
based on what was passed to it. In practice the only way to safely
use it is for the caller to always pass nbarg-1 and make that last
entry point to the last arg + its strlen. This is annoying because
it makes it as painful to use as the infamous strncpy() while it has
all the information the caller needs.

This patch changes its behavior so that it guarantees that at least
one argument will point to the trailing zero at the end of the output
string, as long as there is at least one argument. The caller just
has to pass +1 to the arg count to make sure at least a last one is
empty.