- 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)

like updwtmpx

So /proc/<uid>/loginuid will be filled by the uid of the user.
This will fix entries in 'last' with "gone - no logout" instead of
'still logged in'

use g_*

Follow coding standard

AC_GNU_SOURCE is replaced with AC_USE_SYSTEM_EXTENSIONS (available
from autoconf 2.6.0)

This conflicts with struct exit_status in <utmp.h>

If we're not compiling with USE_UTMP, these defines will not
be available

The utmp record is generally looked up by the ut_id field. Setting
this field means we can use a blank username and host when the
session exits

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.

g_strncpy() is the wrong function for copying strings in struct utmp[x]
as it always terminates strings.

strncpy() itself would be a good choice, but is marked by many compilers
as being unsafe to use.

str2memcpy() is taken from util-linux, and is exactly right for this
application.