Refactor: libcrmcommon, fencer: Fix some option defaults

The default value for PCMK_STONITH_HOST_MAP and PCMK_STONITH_HOST_LIST
is NULL (unset), not the empty string.

The default for PCMK_STONITH_HOST_CHECK is more complicated.

PCMK_OPT_DC_VERSION doesn't have a meaningful default (it's not the
literal string PCMK_VALUE_NONE).

PCMK_OPT_CLUSTER_INFRASTRUCTURE's default value depends on what cluster
stacks are supported.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>