login: Introduce matches_last_entry to utmp processing

This simplifies internal_getut_nolock and fixes a regression,
introduced in commit be6b16d9
("login: Acquire write lock early in pututline [BZ #24882]")
in pututxline because __utmp_equal can only compare process-related
utmp entries.

Fixes: be6b16d9
Change-Id: Ib8a85002f7f87ee41590846d16d7e52bdb82f5a5