Add a new function buffer_add_n(), and the corresponding

function-like macro OSRF_BUFFER_ADD_N.

These facilities append a specified number of characters from
an input string to a growing_buffer.  They are intended for
situations where the length of the input string is already known,
or where the input string is not nul-terminated in the right
place.

git-svn-id: svn://svn.open-ils.org/OpenSRF/trunk@1568 9efc2488-bf62-4759-914b-345cdb29e865