Add an XSI replacement for func_split_short_opt, with test cases.

Add an XSI replacement for func_split_short_opt, with test cases.

* libltdl/config/getopt.m4sh (m4go_shortnoargs): Remove 'v' now
that getopt.m4sh doesn't steal that letter for the --version
short option.
* libltdl/config/ltmain.m4sh (M4SH_GETOPTS): We can now use 'v'
as a short option for '--verbose'.
* libltdl/config/getopt.m4sh (m4go_printopts): Add back in the
leading '-' for remaining non-argument short options.
(func_split_short_opt): Use the correct sed script variable.
* libltdl/m4/libtool.m4 (_LT_CHECK_SHELL_FEATURES): Also ensure
that XSI substring extraction is supported.
(_LT_PROG_XSI_SHELLFNS): Use XSI substring extraction to split
short options to avoid unnecessary forks if the host shell
supports that.
* tests/getopt-m4sh.at: New tests for getopt.m4sh coverage.
* Makefile.am (TESTSUITE_AT): Add getopt-m4sh.at.
(TESTS_ENVIRONMENT): Pass M4SH and SED for substition into the
m4sh expansion performed in tests/getopt-m4sh.at.

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