config: use config checks and add support for creating trees from config definition (config_def_create_tree fn)

Configuration checking is initiated during config load/processing
(_process_config fn) which is part of the command context
creation/refresh.

This patch also defines 5 types of trees that could be created from
the configuration definition (config_settings.h), the cfg_def_tree_t:

  - CFG_DEF_TREE_CURRENT that denotes a tree of all the configuration
    nodes that are explicitly defined in lvm.conf/--config

  - CFG_DEF_TREE_MISSING that denotes a tree of all missing
    configuration nodes for which default valus are used since they're
    not explicitly used in lvm.conf/--config

  - CFG_DEF_TREE_DEFAULT that denotes a tree of all possible
    configuration nodes with default values assigned, no matter what
    the actual lvm.conf/--config is

  - CFG_DEF_TREE_NEW that denotes a tree of all new configuration nodes
    that appeared in given version

  - CFG_DEF_TREE_COMPLETE that denotes a tree of the whole configuration
    tree that is used in LVM2 (a combination of CFG_DEF_TREE_CURRENT +
    CFG_DEF_TREE_MISSING). This is not implemented yet, it will be added
    later...

The function that creates the definition tree of given type:

  struct dm_config_tree *config_def_create_tree(struct config_def_tree_spec *spec);

Where the "spec" specifies the tree type to be created:

  struct config_def_tree_spec {
    cfg_def_tree_t type;	/* tree type */
    uint16_t version;		/* tree at this LVM2 version */
    int ignoreadvanced;		/* do not include advanced configs */
    int ignoreunsupported;	/* do not include unsupported configs */
  };

This tree can be passed to already existing functions that write
the tree on output (like we already do with cmd->cft).

There is a new lvm.conf section called "config" with two new options:

  - config/checks which enables/disables checking (enabled by default)

  - config/abort_on_errors which enables/disables aborts on any type of
    mismatch found in the config (disabled by default)

1 files changed, 14 insertions, 1 deletions

diff --git a/doc/example.conf.in b/doc/example.conf.in
index ab83c0c7c..9670b92fe 100644
--- a/doc/example.conf.in
+++ b/doc/example.conf.in
@@ -10,6 +10,20 @@
 # N.B. Take care that each setting only appears once if uncommenting
 # example settings in this file.
 
+# This section allows you to set the way the configuration settings are handled.
+config {
+
+    # If enabled, any LVM2 configuration mismatch is reported.
+    # This implies checking that the configuration key is understood
+    # by LVM2 and that the value of the key is of a proper type.
+    # If disabled, any configuration mismatch is ignored and default
+    # value is used instead without any warning (a message about the
+    # configuration key not being found is issued in verbose mode only).
+    checks = 1
+
+    # If enabled, any configuration mismatch aborts the LVM2 process.
+    abort_on_errors = 0
+}
 
 # This section allows you to configure which block devices should
 # be used by the LVM system.
@@ -359,7 +373,6 @@ shell {
 
 # Miscellaneous global LVM2 settings
 global {
-
     # The file creation mask for any files and directories created.
     # Interpreted as octal if the first digit is zero.
     umask = 077