parse-helpers: allow port 0 for socket bind items

This patch adds a new parameter to parse_ip_port_range, giving callers
the option to allow ranges to have their min be 0 instead of 1.

This is then used by parse_ip_ports_token, intern used by
parse_socket_bind_item to allow port 0 when restricting bind system
calls with SocketBindDeny / SocketBindAllow.

With this, users running server software written using the golang
standard library will be able to effectively sandbox their software,
albeit with a small loss in security protections by allowing the
process to bind on a random port in the
/proc/sys/net/ipv4/ip_local_port_range.

src/basic/parse-util.c

@@ -691,7 +691,7 @@ int parse_ip_port(const char *s, uint16_t *ret) {
         return 0;
 }
 
-int parse_ip_port_range(const char *s, uint16_t *low, uint16_t *high) {
+int parse_ip_port_range(const char *s, uint16_t *low, uint16_t *high, bool allow_zero) {
         unsigned l, h;
         int r;
 
@@ -699,7 +699,10 @@ int parse_ip_port_range(const char *s, uint16_t *low, uint16_t *high) {
         if (r < 0)
                 return r;
 
-        if (l <= 0 || l > 65535 || h <= 0 || h > 65535)
+        if (l > 65535 || h > 65535)
+                return -EINVAL;
+
+        if (!allow_zero && (l == 0 || h == 0))
                 return -EINVAL;
 
         if (h < l)

src/basic/parse-util.h

@@ -139,7 +139,7 @@ int parse_fractional_part_u(const char **s, size_t digits, unsigned *res);
 int parse_nice(const char *p, int *ret);
 
 int parse_ip_port(const char *s, uint16_t *ret);
-int parse_ip_port_range(const char *s, uint16_t *low, uint16_t *high);
+int parse_ip_port_range(const char *s, uint16_t *low, uint16_t *high, bool allow_zero);
 
 int parse_ip_prefix_length(const char *s, int *ret);