Improve test framework: create_file

create_file '-' sends to standard output
create_file --label=LABEL
create_file passes other options to tfw_createfile
Refactor tests: testdefs_java.sh, testdefs_meshms.sh

meshms_add_messages() now puts message texts into TEXT[$n] array
instead of "text$n" files
Fix HTTP Authorization header parsing

Was returning status 500 if Authorization: Basic token was long, because
_reserve() did not have enough free space at start of buffer.  The
solution was to _commit(r) before attempting the _reserve_str() calls
for the decoded username and password.

As a separate issue, the status 500 also triggered an assertion failure
because http_request_parse_header() was returning 400 but
response.result_code had already been set to 500.
MeshMS Java API: list conversations
Refactor Java JSON parsing into JSONTokeniser
MeshMS Java API: list messages