getpt.3, posix_openpt.3, pts.4: Use the term "pseudoterminal multiplexor device" for /dev/ptmx

Let's use some consistent terminology for this device.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man3/getpt.3

@@ -18,7 +18,7 @@ getpt \- open a new pseudoterminal master
 .BR getpt ()
 opens a new pseudoterminal device and returns a file descriptor
 that refers to that device.
-It is equivalent to
+It is equivalent to opening the pseudoterminal multiplexor device
 .PP
 .in +4n
 .EX
@@ -26,7 +26,7 @@ open("/dev/ptmx", O_RDWR);
 .EE
 .in
 .PP
-on Linux systems, though the pseudoterminal master is located
+on Linux systems, though the pseudoterminal multiplexor device is located
 elsewhere on some systems that use the GNU C library.
 .SH RETURN VALUE
 .BR getpt ()

man3/posix_openpt.3

@@ -95,7 +95,7 @@ is part of the UNIX 98 pseudoterminal support (see
 .SH NOTES
 Some older UNIX implementations that support System V
 (aka UNIX 98) pseudoterminals don't have this function, but it
-is easy to implement:
+can be easily implemented by opening the pseudoterminal multiplexor device:
 .PP
 .in +4n
 .EX

man4/pts.4

@@ -11,6 +11,7 @@ ptmx, pts \- pseudoterminal master and slave
 .SH DESCRIPTION
 The file
 .I /dev/ptmx
+(the pseudoterminal multiplexor device)
 is a character file with major number 5 and
 minor number 2, usually with mode 0666 and ownership root:root.
 It is used to create a pseudoterminal master and slave pair.