opt: add an int decrementing helper function

opt_dec_intval decrements an int value, just as opt_inc_intval
increments.

There is not much more to say, other than it allows this
kind of thing, with balanced opposing options:

    static int opt_verbosity = 0;
    static struct opt_table options[] = {
            OPT_WITHOUT_ARG("-q|--quiet", opt_dec_intval,
                            &opt_verbosity, "print less"),
            OPT_WITHOUT_ARG("-v|--verbose", opt_inc_intval,
                            &opt_verbosity, "print more"),
            OPT_ENDTABLE
    };

which is an occasionally seen idiom.  It allows, e.g., people who like
quiet to use `alias foo='foo -q'`, while letting them get back to
normal and verbose modes with various amounts of '-v's.
Signed-off-by: Douglas Bagnall <douglas@halo.gen.nz>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

ccan/opt/helpers.c

@@ -165,6 +165,12 @@ char *opt_inc_intval(int *i)
 	return NULL;
 }
 
+char *opt_dec_intval(int *i)
+{
+	(*i)--;
+	return NULL;
+}
+
 /* Display version string. */
 char *opt_version_and_exit(const char *version)
 {

ccan/opt/opt.h

@@ -450,8 +450,9 @@ void opt_show_ulonglongval_si(char buf[OPT_SHOW_LEN], const unsigned long long *
 
 
 
-/* Increment. */
+/* Increment and decrement. */
 char *opt_inc_intval(int *i);
+char *opt_dec_intval(int *i);
 
 /* Display version string to stdout, exit(0). */
 char *opt_version_and_exit(const char *version);