Fixed "sendmsg() failed" alerts on HP-UX.

HP-UX needs _HPUX_ALT_XOPEN_SOCKET_API to be defined to be able to
use various POSIX versions of networking functions.  Notably sendmsg()
resulted in "sendmsg() failed (9: Bad file number)" alerts without it.

See xopen_networking(7) for more details.

auto/os/conf

diff --git a/auto/os/conf b/auto/os/conf
index 5e658b5..fe72016 100644
--- a/auto/os/conf
+++ b/auto/os/conf
@@ -48,6 +48,7 @@
         CORE_DEPS="$UNIX_DEPS $POSIX_DEPS"
         CORE_SRCS="$UNIX_SRCS"
         CC_AUX_FLAGS="$CC_AUX_FLAGS -D_XOPEN_SOURCE -D_XOPEN_SOURCE_EXTENDED=1"
+        CC_AUX_FLAGS="$CC_AUX_FLAGS -D_HPUX_ALT_XOPEN_SOCKET_API"
     ;;
 
     OSF1:*)

src/os/unix/ngx_posix_config.h

diff --git a/src/os/unix/ngx_posix_config.h b/src/os/unix/ngx_posix_config.h
index 4d432a7..4cf90cc 100644
--- a/src/os/unix/ngx_posix_config.h
+++ b/src/os/unix/ngx_posix_config.h
@@ -12,6 +12,7 @@
 #if (NGX_HPUX)
 #define _XOPEN_SOURCE
 #define _XOPEN_SOURCE_EXTENDED  1
+#define _HPUX_ALT_XOPEN_SOCKET_API
 #endif