Skip filesystem checks for abstract namespace unix sockets

[afonsojanu]

Fixes #2699.

Linux supports abstract namespace unix sockets, whose address starts with a null byte and has no presence on the filesystem. Binding gunicorn to one currently crashes before the server starts:

bind = "unix:\0my-socket"

ValueError: embedded null byte

UnixSocket.__init__ always calls os.stat() on the address to check for and remove a stale socket file from a previous run. os.stat() rejects any path with an embedded null byte outright, so it never has a chance to return ENOENT the way a genuinely missing path would, and the ValueError propagates straight out of the constructor.

bind() has the same problem a step later: it unconditionally chowns the socket path, which would fail the same way once the stat call above stops being in the way.

Since abstract sockets don't exist on the filesystem, there's nothing to stat, remove, or chown for them, so both operations are skipped when the address starts with a null byte.

Added tests covering:

• the abstract-namespace case skips os.stat()
• a normal filesystem path still goes through the existing stat/remove logic
• bind() skips chown() for an abstract-namespace address

Ran the full test suite plus pylint and pycodestyle on the touched files, all clean.

[benoitc]

Thanks @afonsojanu, merged.