Refactor: libcrmcommon: put exit/return handling in own source/header files

Before 57d48a1b, API clients (e.g. sbd) could include crm/common/util.h without
including crm/crm.h.

To preserve that, and to more logically group things, all code related to
handling function return values and exit status codes is now grouped in its own
source files (crm/common/results.h and lib/common/results.c). The new header is
included by util.h. The old header crm/error.h is gone.