Adjusted ENOBUFS logging level. On local datagram sockets ENOBUFS might legitimately appear due to insufficient space in socket buffers, as seen on BSD systems. Accordingly, ngx_connection_error() now logs it with logging level set for the particular connection errors, either "info" or "error" (instead of "alert", which was previously used).

--- a/src/os/unix/ngx_errno.h
+++ b/src/os/unix/ngx_errno.h
@@ -55,6 +55,7 @@ typedef int               ngx_err_t;
 #define NGX_ELOOP         ELOOP
 #define NGX_EBADF         EBADF
 #define NGX_EMSGSIZE      EMSGSIZE
+#define NGX_ENOBUFS       ENOBUFS
 
 #if (NGX_HAVE_OPENAT)
 #define NGX_EMLINK        EMLINK