MINOR: server: distinguish name references with new SRV_F_NAME_REFD flag

MINOR: server: distinguish name references with new SRV_F_NAME_REFD flag

Until now, every form of "this server is referenced by something in the
running config" was collapsed onto a single flag, SRV_F_NON_PURGEABLE,
which prevents the server from being removed via 'del server'. This
catches everything but conflates two distinct properties:

  - the server object itself is pinned by another runtime structure
    (e.g. DNS resolution attached to it), versus
  - the server's *name* is referenced statically (use-server rules,
    track chains, sample-fetch arguments of type ARGT_SRV)

These differ for any operation that touches the name but not the
object identity, e.g. the runtime rename feature added next. Removing
a name-referenced server is still forbidden (the rule text would
dangle), but renaming such a server should also be forbidden for the
same reason - while renaming a resolver-pinned server is fine, since
the resolver holds the object pointer and doesn't care about the name.

Introduce SRV_F_NAME_REFD for the name-reference case and move the
three name-based setters (sample.c ARGT_SRV resolution, proxy.c
use-server resolution, server.c track chain setup) from
SRV_F_NON_PURGEABLE to SRV_F_NAME_REFD. The resolvers.c call site
keeps SRV_F_NON_PURGEABLE since it is the object-pinned case.

Adjust 'del server' to check both flags so the set of servers it
refuses to remove is unchanged: same observable behavior, just a
richer internal taxonomy.

A subsequent patch introducing 'set server name' will gate on
SRV_F_NAME_REFD only.