refactor: rename find_buffer to get_buffer.

* lib/zile/buffer.lua, lib/zile/funcs.lua,
lib/zmacs/zlisp/buffer.zl, lib/zmacs/zlisp/file.zl,
lib/zmacs/zlisp/help.zl, lib/zmacs/zlisp/lisp.zl,
lib/zz/commands/buffer.lua, lib/zz/commands/file.lua,
lib/zz/commands/help.lua (find_buffer): Rename to
get_buffer.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>