conf: Don't pass leading ~ to parse_port_range() on exclusions

Commit 84fec4e998 ("Clean up parsing of port ranges") drops the
strspn() call before the parsing of excluded port ranges, because now
we're checking against any stray characters at every step.

However, that also has the effect of passing ~ as first character to
the new parse_port_range(), which makes no sense: we already checked
that ~ is the first character before the call, so skip it.

Alona reported this output:
  Invalid port specifier ~15000,~15001,~15006,~15008,~15020,~15021,~15090

while the whole specifier is indeed valid.

Reported-by: Alona Paz <alkaplan@redhat.com>
Fixes: 84fec4e998 ("Clean up parsing of port ranges")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

conf.c

@@ -262,6 +262,7 @@ static int conf_ports(const struct ctx *c, char optname, const char *optarg,
 			exclude_only = false;
 			continue;
 		}
+		p++;
 
 		if (parse_port_range(p, &p, &xrange))
 			goto bad;