tmac: introduce real manual reference macro instead of overloading IR

The overloading of IR emits magic \X'...' sequences that turn into HTML manual links.
But not all such IR invocations should be manual links;
those had to be written to avoid the IR macro before.
Worse, the \X'...' ending the IR causes troff to emit only a single space after a period.

Defining a new IM macro for manual references fixes both problems.

Fixes #441.

man/man1/scat.1

@@ -10,7 +10,7 @@ outside the solar system
 and implements database-like manipulations
 on sets of such objects.
 It also provides an interface to
-.IR astro (1)
+.IM astro (1)
 to plot the locations of solar system objects.
 Finally, it displays images from the
 Space Telescope Science Institute's
@@ -119,7 +119,7 @@ The names
 and
 .B comet
 refer to the earth's penumbra at lunar distance and the comet installed in the current
-.IR astro (1).
+.IM astro (1) .
 The output is the planet's name, right ascension and declination, azimuth and altitude, and phase
 for the moon and sun, as shown by
 .BR astro .
@@ -182,7 +182,7 @@ collects all objects in the patches that cover the current set.
 .TP
 .BI astro " option"
 Run
-.IR astro (1)
+.IM astro (1)
 with the specified
 .I options
 (to which will be appended
@@ -324,7 +324,7 @@ Draw a map of the Pleiades.
 .SH SOURCE
 .B \*9/src/cmd/scat
 .SH SEE ALSO
-.IR astro (1)
+.IM astro (1)
 .br
 .B \*9/sky/constelnames\ \ 
 the three-letter abbreviations of the constellation names.