Update to 2.4.2

Changelog:
Noteworthy changes in version 2.4.2 (2015-12-02) [C7/A7/R2]
------------------------------------------------

* The nPth version of the connect system hook does now wrap the call
  with npth_unprotec/npth_protect to avoid blocking during a connect.

* Add feature to assuan_sock_connect_byname to test for SOCKS5
  availability.

Noteworthy changes in version 2.4.1 (2015-11-23) [C7/A7/R1]
------------------------------------------------

* In Tor mode fallback to port 9150 if 9050 is not listening.

* Allow building with older mingw-w64 versions.

Noteworthy changes in version 2.4.0 (2015-11-03) [C7/A7/R0]
------------------------------------------------

* New flags "socks" and "tor-mode" for assuan_sock_{set,get}_flag.

* New function assuan_sock_connect_byname.

* Require at least libgpg-error 1.17.

* Interface changes relative to the 2.3.0 release:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
assuan_sock_connect_byname    NEW.
ASSUAN_SOCK_TOR              NEW.
ASSUAN_SOCK_SOCKS            NEW.
assuan_sock_set_flag          EXTENDED.
assuan_sock_get_flag          EXTENDED.

Noteworthy changes in version 2.3.0 (2015-08-28) [C6/A6/R0]
------------------------------------------------

* Now wipes out the memory of the context structure before freeing.
  The context may have stored sensitive data in its line buffers.

* Fixed a problem with the data length limit in assuan_inquire.

* Returns GPG_ERR_SOURCE_ASSUAN with errors from functions w/o a
  context.

* Two new functions to tweak the behaviour of the socket wrappers.

* Experimental code to support Cygwin's local sockets.

* By default build without a build timestamp.

* Interface changes relative to the 2.2.1 release:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
assuan_sock_set_flag    NEW.
assuan_sock_get_flag    NEW.

(ryoon)