Handling ENOBUFS error same was as EAGAIN.

Unlike EAGAIN, ENOBUFS returned on OSX when trying to send many relatively
small (64 bytes) fragments.

Found during investigation of #167 issue on GitHub.

src/nxt_errno.h

@@ -45,6 +45,7 @@ typedef int                        nxt_err_t;
 #define NXT_EILSEQ                 EILSEQ
 #define NXT_ETIME                  ETIME
 #define NXT_ENOMOREFILES           0
+#define NXT_ENOBUFS                ENOBUFS
 
 #if (NXT_HPUX)
 /* HP-UX uses EWOULDBLOCK instead of EAGAIN. */