MINOR: hbuf: new lightweight hbuf API

MINOR: hbuf: new lightweight hbuf API

Add a new lightweight hbuf API to buffer formatted strings, similar to the
existing buffer API (struct buffer), extracting the code which already does this
in haterm_init.c. This is used by haterm to build its configuration in memory
(fileless mode). And this will be used by haload to do the same thing.

Update haterm to use this new API.

Note: hstream_str_buf_append() has been renamed to hbuf_str_append().