Add some missing MAPI prototypes to mapi.h (spotted by winapi_check)
and include it where they are implemented.
Make mapi.h and mapix.h compatible. gcc does not like macros and
prototypes to be redefined so this involved protecting against
multiple definitions.
Fix the MAPISendMail() implementation prototype.

Implement MAPISendMail, stub MAPILogoff and make the stubs for
MAPIInitialize, MAPILogon{,Ex} and MAPILogoff return success.