Implement the --ycpc option

The --ycpc option makes Y2R use a different ycpc interpreter. This is
necessary because as we develop Y2R we also tweak ycpc as needed and
this modified version usually won't be installed in $PATH.
Recognize Y2R_YCPC, Y2R_MODULE_PATH and Y2R_INCLUDE_PATH

These variables allow to avoid specifying the --ycpc, --module-path or
--include-path options all the time.
Recognize Y2R_YCPC, Y2R_MODULE_PATH and Y2R_INCLUDE_PATH also in tests
Add YCP module stubs for specs and use them by default

These stubs allow us to be independent on YCP modules installed on the
system, which may be incompatible with used ycpc interpreter,
incomplete, etc.