Remove struct buffer

I've long wanted to remove 'struct buffer', and thanks to Simon's
earlier patch, I was finally able to do so.  My feeling has been that
gdb already has several decent structures available for growing
strings: std::string of course, but also obstack and even objalloc
from BFD and dyn-string from libiberty.  The previous patches in this
series removed all the uses of struct buffer, so this one can remove
the code and the remaining #includes.