ucm: add LibraryConfig support

This commit allows to define private alsa-lib's configuration. When
the configuration is present, the device values ("PlaybackCTL",
"CaptureCTL", "PlaybackMixer", "CaptureMixer", "CapturePCM")
are prefixed with '_ucmHEXA.' string where HEXA is replaced by the
unique hexadecimal number identifying the opened ucm manager handle.

    Syntax 4

    LibraryConfig.a_label.SubstiConfig {
            # substituted library configuration like:
            usr_share_dir "${ConfLibDir}"
    }

    LibraryConfig.b_label.Config {
            # non-substituted library configuration like:
            usr_share_dir "/usr/share/alsa"
    }

    The File counterparts:

    LibraryConfig.c_label.SubstiFile "/some/path"
    LibraryConfig.d_label.File "/some/path"

Note that for files the contents is substituted on the request,
but the file name is always substituted (useful for ${ConfDir} etc.).

The private configuration is not saved or preserved. It's life time
belongs to the opened ucm manager handle.

Signed-off-by: Jaroslav Kysela <perex@perex.cz>

include/use-case.h

@@ -257,6 +257,7 @@ int snd_use_case_get_list(snd_use_case_mgr_t *uc_mgr,
  *   - NULL 		- return current card
  *   - _verb		- return current verb
  *   - _file		- return configuration file loaded for current card
+ *   - _alibcfg		- return private alsa-lib's configuration for current card
  *
  *   - [=]{NAME}[/[{modifier}|{/device}][/{verb}]]
  *                      - value identifier {NAME}