Tinkering with macros.

Tinkering with macros.

1. In OSRF_BUFFER_ADD, OSRF_BUFFER_ADD_CHAR, and OSRF_BUFFER_RESET:
eliminated multiple evaluations of macro arguments.

2. In OSRF_BUFFER_ADD: renamed local variable __tl to _tl, since
identifiers beginning with two underscores are reserved.

3. In OSRF_BUFFER_RESET: applied the do/while(0) trick so that the
macro will be work as intended when subject to an "if".

4. Added new macro OSRF_BUFFER_C_STR to return a const pointer to
the internal buffer of a growing_buffer.  This new macro will enable
safe and direct access to the buffer contents without violating
encapsulation and without incurring a malloc and free.

git-svn-id: svn://svn.open-ils.org/OpenSRF/trunk@1572 9efc2488-bf62-4759-914b-345cdb29e865