Implemented write_buf() wrapper. Still needs fuse_buf_copy().

Made alterations to read_buf() wrapper to use an array, instead of a
single item, to accommodate different strategies.

Altered loopback.pl to support write_buf, and to use the modified
read_buf semantics.

Also changed open() use in daemonize() inside loopback.pl to 3-operand
calling syntax, because it's just the right thing to do.