bt(5)/btrace(8): add support for str()

bt(5)/btrace(8): add support for str()

Implement initial support for the str() function, which is used
primarily to truncate or NUL-terminate strings from either cli args
or args to tracepoints and syscalls.

Current implementation only supports cli args and is primarily for
compatability with bpftrace. Future work is needed once dt(4)
supports builtin args other than long values.

Adds a regress test and wires in argument-based tests again.

ok mpi@