Move bd_is_plugin_available function's prototype to the right place

It's part of the plugin-querying API, so it should be in the plugins.h file.

Also remove the deprecated function providing list of available plugins as a
list of enums -- it'd be really hard to work with.
Add new functions required from the DM plugin

There are 5 functions that blivet uses from the pyblock and which can be
implemented in the DM plugin instead allowing blivet dropping its dependency on
pyblock.

Also fix some functions' names.