Split OpamClient content

[rjbou]

OpamClient module is more and more huge, it contains in its first part all opam commands first level function, and in its second part only opam init functions (~700 lines on a ~1900 lines file). Initialisation functions are more and more huge with Windows support (Cygwin, Git, MSYS2, etc.).
It can be separated into OpamClient and OpamClientInit, the second module/file containing only init related functions. We can keep OpamClient.init functions that is just a redirection to OpamClientInit.init to not break all projects using opam libraries.