Expand default-no-install prog list in ./configure --help output,

Expand default-no-install prog list in ./configure --help output,
and fix some []-quoting bugs in sed expressions.
* configure.ac: Hard-code the list, "arch,su" here as well
as in src/Makefile.am, and ensure the two stay in sync.
* m4/include-exclude-prog.m4 (gl_INCLUDE_EXCLUDE_PROG): Use $2,
rather than the nearly-equivalent shell variable.
Karel Zak reported that ./configure --help's output included
the literal string, $gl_no_install_progs_default.