Use dns_fixedname_initname() where possible

Use dns_fixedname_initname() where possible

Replace dns_fixedname_init() calls followed by dns_fixedname_name()
calls with calls to dns_fixedname_initname() where it is possible
without affecting current behavior and/or performance.

This patch was mostly prepared using Coccinelle and the following
semantic patch:

    @@
    expression fixedname, name;
    @@
    -	dns_fixedname_init(&fixedname);
    	...
    -	name = dns_fixedname_name(&fixedname);
    +	name = dns_fixedname_initname(&fixedname);

The resulting set of changes was then manually reviewed to exclude false
positives and apply minor tweaks.

It is likely that more occurrences of this pattern can be refactored in
an identical way.  This commit only takes care of the low-hanging fruit.