tree-wide: get rid of strappend()

It's a special case of strjoin(), so no need to keep both. In particular
as typing strjoin() is even shoert than strappend().

src/nspawn/nspawn-network.c

@@ -489,7 +489,7 @@ int setup_macvlan(const char *machine_name, pid_t pid, char **ifaces) {
                 if (r < 0)
                         return log_error_errno(r, "Failed to add netlink interface index: %m");
 
-                n = strappend("mv-", *i);
+                n = strjoin("mv-", *i);
                 if (!n)
                         return log_oom();
 
@@ -564,7 +564,7 @@ int setup_ipvlan(const char *machine_name, pid_t pid, char **ifaces) {
                 if (r < 0)
                         return log_error_errno(r, "Failed to add netlink interface index: %m");
 
-                n = strappend("iv-", *i);
+                n = strjoin("iv-", *i);
                 if (!n)
                         return log_oom();
 

src/nspawn/nspawn-settings.c

@@ -505,7 +505,7 @@ int config_parse_network_zone(
         assert(lvalue);
         assert(rvalue);
 
-        j = strappend("vz-", rvalue);
+        j = strjoin("vz-", rvalue);
         if (!ifname_valid(j)) {
                 log_syntax(unit, LOG_ERR, filename, line, 0, "Invalid network zone name, ignoring: %s", rvalue);
                 return 0;

src/nspawn/nspawn.c

@@ -725,7 +725,7 @@ static int parse_argv(int argc, char *argv[]) {
                 case ARG_NETWORK_ZONE: {
                         char *j;
 
-                        j = strappend("vz-", optarg);
+                        j = strjoin("vz-", optarg);
                         if (!j)
                                 return log_oom();