config: add support for configuration check (config_def_check fn)

config: add support for configuration check (config_def_check fn)

Add support for configuration checking - type checking and recognition
of registered configuration settings that LVM2 understands and also
check the structure of the configuration. Log error on any mismatch
found.

A hash over all allowed configuration paths is created which helps
with matching the exact configuration (lvm.conf/--config tree) with
the configuration item definition from config_settings.h in an
efficient and one-step way.

Two more helper flags are introduced for each configuration definition
item:

  - CFG_USED which marks the item as being used (lvm.conf/--config)
    This helps with identifying missing configuration options
    (and for which defaults were used) when traversing the tree later.

  - CFG_VALID which denotes that the item has already been checked and
    it was found valid. This improves performance, so if the check
    is called once again on the same tree which was not reloaded, we
    can just return the state from previous check (with a possibility
    to force the check if needed).

The new function that config.h exports and which is going to be used
to perform the configuration checking is:

  int config_def_check(struct cmd_context *cmd, int force, int skip, int suppress_messages)

...which is exported internally via config.h.