Add posix::basic_buffer template and posix::buffer class

posixx::basic_buffer provides easy to use, STL-container-lke and
exception-safe class to manipulate buffers. It's a pseudo-static array
(you can resize it but you can't append or insert/remove arbitrary
elements). It's parametrized to support different kinds of allocators (not
in the STL-sense, it uses a realloc-like function).

posix::buffer is just a posix::basic_buffer< &realloc >.