Big loginrec.c update

Big loginrec.c update
- MIN_SIZEOF macro to pick smallest buffer for str[ln]cpy()
  - fixes lastlog clobber spotted by P.S.S.Camp <P.S.S.Camp@ukc.ac.uk>
- Added some top-level api documentation
- Re-ordered construct_utmp(x)() to build well-formed structures for
   both login and logout, based on li->type.
  - change wtmp(x)_logout() to match new construct_*() form
- Improved error responses throughout
- Add ut_type cross-check for 'lastlog-from-wtmp(x)' where available
- utmp_write_direct() has had some testing. Oh yes!
- Re-ordered loginrec.h slightly