MINOR: server: rename <all_servers> list to <servers_to_init>

MINOR: server: rename <all_servers> list to <servers_to_init>

Now that <all_servers> is only used during postparsing, it is better to
rename it <servers_to_init> to reflect its real purpose.

The attach point in struct server is also renamed to <el_alt> for
"alternative attach point". This is a generic name as it will be reused
for another list storage during runtime.