Formatting tweak: balance () with m4sh case statements.

Formatting tweak: balance () with m4sh case statements.

* lib/m4sugar/m4sh.m4 (_AS_DETECT_BETTER_SHELL)
(_AS_SHELL_SANITIZE, _AS_ECHO_N_PREPARE, _AS_ECHO_PREPARE)
(AS_SET_CATFILE, _AS_TEST_PREPARE): Add strategic shell comments.
(_AS_CASE, _AS_CASE_DEFAULT, AS_CASE): Rearrange newlines, to
allow output of strategic shell comments.
(AS_VERSION_COMPARE): Use AS_CASE.
* tests/m4sh.at (AS@&t@_IF and AS@&t@_CASE): Enhance test.

Signed-off-by: Eric Blake <ebb9@byu.net>