[test_server,common_test] Fix cross cover mechansim

Update the interface for cross cover analysis (collection of cover
data over multiple tests) so it can be used via common_test and
directly with test server.

The concept of 'application' in the cross cover interface is removed
and replaced with an arbitrary Tag=atom() which identifies a test run.
[test_server] Add test of code cover mechanism
[test_server] Update documentation about cross cover

Add info about cross statement in cover file:
{cross,[{Tag,Modules}]}.

Add parameter Tests = [{Tag,Dir}] to test_server_ctrl:cross_cover_analyse
[common_test] Add test case for cross cover mechanism
[common_test] Add documentation for cross cover analysis
Add missing error handling clause for handle_request packet return

This should have been included in commit 91a223d3.
Fix timing of service start event

Whether or not it came first was a matter of timing. Originally bungled
in commit 476db530.
crypto: Document all types currently available for hmac_init
crypto: Provide a generic interface for HMAC generation
crypto: Add RFC-2202 test vectors for HMAC-MD5 and HMAC-SHA1
crypto: Add test cases for the generic hmac interface in the RFC-4231 tests
crypto: Add RFC-4231 test vectors for output truncation
[test_server] Don't use print/3 in cross_cover_analyse

Cross cover analysis is executed outside the scope of
test_server_ctrl, i.e. test_server_ctrl is not necessarily
started. This may cause test_server_ctrl:print/3 to hang. Instead use
io:fwrite directly (as it was before).
wx: Build targets to platform subdirs

And add code so wx can be run from src tree during development.
asn1_SUITE: Only run testExtensionAdditionGroup/1 once
testSetOptional: Correct test case for decoding of corrupt data

Eliminate the spawning of the process since the original bug is
fixed.
Simplify testSetOptional by introducing a roundtrip/2 function
Simplify testChoExtension by introducing a roundtrip/2 function
Simplify testSeqExtension.erl by introducing a roundtrip/2 function

Make the test suite more readable by introducing and using a
roundtrip/2 function.
Simplify testChoRecursive by introducing a roundtrip/2 function