RFC: Set vl.host in plugin_dispatch_values()

[octo]

Currently, most plugins do something like the following to submit metrics:

value_list_t vl = VALUE_LIST_INIT;
sstrncpy (vl.host, hostname_g, sizeof (vl.host));
plugin_dispatch_values (&vl);

The sstrncpy() line is needed, because the VALUE_LIST_INIT cannot properly initialize the char host[128] field using hostname_g:

plugin.c:158:2: error: missing braces around initializer [-Werror=missing-braces]
  value_list_t vl = VALUE_LIST_INIT;
  ^
plugin.c:158:2: error: (near initialization for ‘vl.host’) [-Werror=missing-braces]
plugin.c:158:2: error: initialization makes integer from pointer without a cast [-Werror]
plugin.c:158:2: error: (near initialization for ‘vl.host[0]’) [-Werror]
plugin.c:158:2: error: initialization makes integer from pointer without a cast [-Werror]

I propose to add code along the lines of:

if (strlen (vl->host) == 0) {
  sstrncpy (vl->host, hostname_g, sizeof (vl->host));
}

to plugin_write_enqueue(), therefore removing the need for ~115 plugins to initialize this field themselves. Eventually, the same technique could be used to set the vl.plugin field, too (see #1949).

[rpv-tomsk]

I see no reasons against this improvement.