gss_accept_sec_context: support reassembling split tokens.

Microsoft will sometimes split GSS tokens when they exceed a certain
size in some protocols.  This is specified in

	[MS-SPNG]: Simple and Protected GSS-API Negotiation
	Mechanism (SPNEGO) Extension

https://winprotocoldoc.blob.core.windows.net/productionwindowsarchives/MS-SPNG/%5bMS-SPNG%5d.pdf

sections 3.1.5.4 to 3.1.5.9.

We extend gss_accept_sec_context() to recognise partial tokens and
to accumulate the fragments until an entire token is available to
be processed.  If the entire token is not yet available,
GSS_S_CONTINUE_NEEDED is returned with a zero length output token.
This is specified in RFC2744 page 25-26 to indicate that no reply
need be sent.

We include updates to the test framework to test split tokens when
using SPNEGO.

tests/gss/check-spnego.in

@@ -138,6 +138,16 @@ ${context} \
     host@host.test.h5l.se || \
     { exitcode=1 ; echo test failed; }
 
+echo "spnego (split tokens)"
+${context} \
+    --token-split=128 \
+    --client-ccache="${cache}" \
+    --mech-type=spnego \
+    --ret-mech-type=krb5 \
+    --name-type=hostbased-service \
+    host@host.test.h5l.se || \
+    { exitcode=1 ; echo test failed; }
+
 echo "test failure cases"
 ${context} --mech-type=ntlm --ret-mech-type=krb5 \
     --client-ccache="${cache}" \