build: add missing fixtures to distribution tarball
doc: update references to current stable branch

The contribution guidelines in CONTRIBUTING.md referred to v0.10 as
the current stable branch, rather than v1.x.
fs: fix readdir on empty directory

`uv_fd_readdir()` should initialize all fields even if there are no
files. Fix the implementation and fix the test that should have been
covered it.
fs: rename uv_fs_readdir to uv_fs_scandir

Closes #1431
doc: document uv_alloc_cb

Fixes #1498
doc: add migration guide from version 0.10
build: add DragonFly BSD support in autotools