In nxt_unit_create() we could leak a mutex created in
nxt_unit_ctx_init().

This could happen if nxt_unit_ctx_init() succeeded but later on we
bailed out of nxt_unit_create(), we would destroy the mutex created in
nxt_unit_create() but not the one created in nxt_unit_ctx_init().

Reorder things so that we do the call to nxt_unit_create() after all the
other checks so if it fails we don't leak the mutex it created.

Co-developed-by: Andrew Clayton <a.clayton@f5.com>
Signed-off-by: Andrew Clayton <a.clayton@f5.com>
Signed-off-by: Alex Colomar <a.colomar@f5.com>

As was reported[0] by @travisbell on GitHub, if running unit from the
terminal in the foreground when hitting ^C to exit it, the ruby
application processes would segfault if they were using threads.

It's not 100% clear where the actual problem lies, but it _looks_ like
it may be in ruby.

The simplest way to deal with this for now is to just ignore SIGINT in
the ruby application processes. Unit will still receive and handle it,
cleanly shutting everything down.

For people who want to handle SIGINT in their ruby application running
under unit they can still trap SIGINT and it will override the ignore.

[0]: https://github.com/nginx/unit/issues/562#issuecomment-1223229585

Closes: https://github.com/nginx/unit/issues/562

The previous commit added/fixed support for abstract Unix domain sockets
on Linux with a leading '@' or '\0'.  To be consistent in all platforms,
treat those prefixes as markers for abstract sockets in all platforms,
and fail if abstract sockets are not supported by the platform.

That will avoid mistakes when copying a config file from a Linux system
and using it in non-Linux, which would surprisingly create a normal socket.