Add discussion of POSIX.1 utmpx specification.

Move some text from CONFORMING To to NOTES.

man5/utmp.5

@@ -219,6 +219,20 @@ removed, record-keeping is turned off.
 .br
 /var/log/wtmp
 .SH "CONFORMING TO"
+.PP
+POSIX.1 does not specify a
+.I utmp
+structure, but rather one named
+.IR utmpx ,
+with specifications for the fields
+.IR ut_type ,
+.IR ut_pid ,
+.IR ut_line ,
+.IR ut_id ,
+.IR ut_user ,
+and
+.IR ut_tv .
+.SS Comparison with Historical Systems
 Linux utmp entries conform neither to v7/BSD nor to System V; they are a
 mix of the two.
 v7/BSD has fewer fields; most importantly it lacks
@@ -244,6 +258,7 @@ such as \fB"new time"\fP in the line field.
 \fBUT_UNKNOWN\fP seems
 to be a Linux invention.
 System V has no \fIut_host\fP or \fIut_addr_v6\fP fields.
+.SH NOTES
 .PP
 Unlike various other
 systems, where utmp logging can be disabled by removing the file, utmp
@@ -256,7 +271,7 @@ Note that the \fIutmp\fP struct from libc5 has changed in libc6.
 Because of this,
 binaries using the old libc5 struct will corrupt
 .IR /var/run/utmp " and/or " /var/log/wtmp .
-.SH NOTES
+.PP
 The file format is machine-dependent, so it is recommended that it be
 processed only on the machine architecture where it was created.
 .PP