buffer: enable tracking of calls to c_str()

buffer: enable tracking of calls to c_str()

Track buffer::ptr::c_str() to catch internal calls that use it, like
buffer::ptr::cmp(). buffer::list::c_str() will be captured by this as
well, since it will do a final buffer::ptr::c_str() and possibly
several more if it needs to rebuild into a single raw buffer.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>