lvmconfig: add --type list and -l|--list

lvmconfig: add --type list and -l|--list

lvmconfig --type list displays plain list of configuration settings.
Some of the existing decorations can be used (--withsummary and
--withversions) as well as existing options/switches (--ignoreadvanced,
--ignoreunsupported, --ignorelocal, --atversion).

For example (displaying only "config" section so the list is not long):

$lvmconfig --type list config
config/checks
config/abort_on_errors
config/profile_dir

$ lvmconfig --type list --withsummary config
config/checks - If enabled, any LVM configuration mismatch is reported.
config/abort_on_errors - Abort the LVM process if a configuration mismatch is found.
config/profile_dir - Directory where LVM looks for configuration profiles.

$ lvmconfig -l config
config/checks - If enabled, any LVM configuration mismatch is reported.
config/abort_on_errors - Abort the LVM process if a configuration mismatch is found.
config/profile_dir - Directory where LVM looks for configuration profiles.

$ lvmconfig --type list --withsummary --withversions config
config/checks - If enabled, any LVM configuration mismatch is reported. [2.2.99]
config/abort_on_errors - Abort the LVM process if a configuration mismatch is found. [2.2.99]
config/profile_dir - Directory where LVM looks for configuration profiles. [2.2.99]

Example with --atversion (displaying global section):

$ lvmconfig --type list global
global/umask
global/test
global/units
global/si_unit_consistency
global/suffix
global/activation
global/fallback_to_lvm1
global/format
global/format_libraries
global/segment_libraries
global/proc
global/etc
global/locking_type
global/wait_for_locks
global/fallback_to_clustered_locking
global/fallback_to_local_locking
global/locking_dir
global/prioritise_write_locks
global/library_dir
global/locking_library
global/abort_on_internal_errors
global/detect_internal_vg_cache_corruption
global/metadata_read_only
global/mirror_segtype_default
global/raid10_segtype_default
global/sparse_segtype_default
global/lvdisplay_shows_full_device_path
global/use_lvmetad
global/thin_check_executable
global/thin_dump_executable
global/thin_repair_executable
global/thin_check_options
global/thin_repair_options
global/thin_disabled_features
global/cache_check_executable
global/cache_dump_executable
global/cache_repair_executable
global/cache_check_options
global/cache_repair_options
global/system_id_source
global/system_id_file

$ lvmconfig --type list global --atversion 2.2.50
global/umask
global/test
global/units
global/suffix
global/activation
global/fallback_to_lvm1
global/format
global/format_libraries
global/segment_libraries
global/proc
global/locking_type
global/wait_for_locks
global/fallback_to_clustered_locking
global/fallback_to_local_locking
global/locking_dir
global/library_dir
global/locking_library