libdmsg - refactor remove hammer2-specific code

* Remove hammer2-specific code.

* Refactor the callback API to allow the hammer service code to handle
  the H2-specific LNK_CONN commands... mostly VOLCONF.
hammer2 - refactor libdmsg volume configuration

* Move the libdmsg volume configuration structures and commands, which
  are hammer2-specific, over to hammer2.
kernel: fix buildkernel by adding include to subr_diskiocom.c

Include vfs/hammer2/hammer2_disk.h to sys/kern/subr_diskiocom.c