POSIX.1 doesn't define ut_host in struct utmpx. Also, Linux has support
for an exit status value in ut_exit. This commit adds conditional code
for both ut_host and ut_exit to maximise portability.
- renamed the two files, including the header was conflicting with official headers
- configure look for utmp/utmpx headers, wo we know which struct to use
- reworked the usage for linux, works mostly (last still showing 'gone' for loggued users)
