(login): Use mutex to secure against concurrent access to wtmp file.
(logout): Rewrite using POSIX calls.
(utmp_fd): Initialized to -1 now. Any value < 0 is treated as closed
in subsequent functions.
(utmp_readonly): New variable, indicating utmp file open for reading
only.
(internal_setutent): New function implementing setutent().
(setutent): Call internal_setutent now.
(endutent): Reset utmp_readonly.
(getutent): Return immediately if utmp file can't be opened.
(getutid): Ditto.
(getutline): Ditto.
(pututline): Ditto. Use mutex to secure against concurrent access to
utmp file.
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