Split out dumper allocation from list insertion

Add a new function, dumper_create(), to allocate a dumper.
dumper_insert() will call this function and retains the existing
behaviour.

This is desirable for performing live dumps of the system. Here, there
is a need to allocate and configure a dumper structure that is invoked
outside of the typical debugger context. Therefore, it should be
excluded from the list of panic-time dumpers.

free_single_dumper() is made public and renamed to dumper_destroy().

Reviewed by: kib, markj
MFC after: 1 week
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D34068