CMake build scripts: prepare for multiple module support.

Generalize CMake build scripts to be able to
easily add new modules (such as box or feeder).
Move prscfg.{h,c} (confetti general library and
header) from third_party/confetti/ to cfg/ to
simplify the build.
Update configuration file templates and parsing
code to use the new confetti.
Rename cfg/tarantool_cfg.h to cfg/warning.h,
to be able to reuse it in warning.c.
Update tests.

Note: tarantool_feeder is now built in mod/feeder;
tarantool_silverbox is built in mod/silverbox.
Modify .gitignore accordingly.