Add func_append test cases for smart and retarded implementations.

Add func_append test cases for smart and retarded implementations.

* tests/getopt-m4sh.at (_LT_AT_FALLBACK_FUNC_EXTRACT): New
macro to extract a fallback function implementation from
ltmain.m4sh.
[option appending]: Use it for a test of the fallback
implementation of func_append.
[enhanced shell option appending]: And also test the enhanced
implementation.
* libltdl/config/ltmain.m4sh (func_append_quoted): Fix missing \
to escape a quoted space for eval.
* libltdl/m4/libtool.m4 (_LT_PROG_XSI_SHELLFNS): Fix missing $ in
dereference of func_append inline replacement for retarded
shells.

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