Fix wording of licensing terms

It has been pointed out that the wording of the license of this library is
not entirely clear.

The term "dual licensing" usually refers to a licence choice of two
licenses "LICENSE1 _or_ LICENSE2.
Instead the license of this library claimed "LICENSE1 _and_ LICENSE2".

After an internal discussion with @metajack and @pasis it was made clear
that the initial idea was to dual license the library in the usual way.

This was also made clear by jack on the ML in the past [0].
As of jack, these licensing terms originated from jquery, which also used
the 'and' version in the past and has since been corrected [1].

This patch changes the license terms to 'MIT or GPLv3' and also adds SPDX
headers [2].

[0] https://groups.google.com/g/libstrophe/c/JkFgr601JQc
[1] https://stackoverflow.com/q/2758409
[2] https://spdx.org

Signed-off-by: Steffen Jaeckel <jaeckel-floss@eyet-services.de>

README.markdown

@@ -5,7 +5,7 @@ libstrophe is a lightweight XMPP client library written in C. It has
 minimal dependencies and is configurable for various environments. It
 runs well on Linux, Unix and Windows based platforms.
 
-libstrophe is dual licensed under MIT and GPLv3.
+libstrophe is dual licensed under MIT or GPLv3.
 
 Build Instructions
 ------------------