syntax-check: fix violations and implement sc_useless_quotes_in_case.

syntax-check: fix violations and implement sc_useless_quotes_in_case.

Contrary to popular belief, Bourne shell does not resplit case
expressions after expansion, so if there are no unquoted shell
metacharacters or whitespace, the quotes are useless.
* cfg.mk (sc_useless_quotes_in_case): New syntax-check rule to
ensure we don't reintroduce useless quoted case expressions.
* build-aux/ltmain.m4sh, m4/libtool.m4, tests/bindir.at,
tests/darwin.at, tests/defs.m4sh, tests/demo-hardcode.test,
tests/demo-nopic.test, tests/link-2.test, tests/quote.test,
tests/sysroot.at: Remove spurious quotes.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>