Improve quoting and use quadrigraphs.

Use double [[ quoting in _AX_AT_CHECK_PATTERN_AWK and use
quadrigraphs for reserved characters.
regen
document github upstream with test suite

Add a comment describing where the upstream is (at github)
and that a test suite is available there.
Per suggestion from Peter Simons.
Add header to AX_AT_DATA_CHECK_PATTERN_AWK
testsuite for AX_AT_DATA_CHECK_PATTERN_AWK

Add testsuite 06-extract.at for AX_AT_DATA_CHECK_PATTERN_AWK(),
including expected output in src/check_pattern.awk.