syntax-check: fix violations and implement sc_prohibit_sed_s_comma.

syntax-check: fix violations and implement sc_prohibit_sed_s_comma.

I like to name temporary directories that I will remove shortly
with two leading commas so that they sort lexicographically at
the top of `ls' output.  Now, `./configure
--prefix=`pwd`/,,inst' works again, for the first time in
several years.
* cfg.mk (sc_prohibit_sed_s_comma): Comma is too common a
character to use routinely as the separator for sed
substitutions on file paths and other variables determined by
the user, causing bugs like the one I describe above.  Make sure
we don't accidentally reintroduce any comma separators in
future.
* Makefile.am, bootstrap, bootstrap.conf, build-aux/extract-trace,
build-aux/general.m4sh, build-aux/git-hooks/commit-msg,
build-aux/git-log-fix, build-aux/ltmain.m4sh, libtoolize.m4sh,
m4/libtool.m4, m4/ltdl.m4, tests/cdemo-undef.test,
tests/cmdline_wrap.at, tests/darwin.at, tests/defs.m4sh,
tests/getopt-m4sh.at, tests/install.at, tests/libtoolize.at,
tests/mdemo/Makefile.am, tests/need_lib_prefix.at,
tests/sysroot.at, tests/tagdemo-undef.test, tests/testsuite.at:
Try to use `|' as the default separator wherever possible,
otherwise something else that doesn't occur in the substitution
expression.
* NEWS: Updated.

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