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/install.1

@@ -15,7 +15,7 @@ cd \*9; ./INSTALL
 .SH DESCRIPTION
 To obtain the Plan 9 tree, use Git
 (see
-.IR git (1))
+.IM git (1) )
 or download a tar file from
 .HR https://9fans.github.io/plan9port "" .
 .PP
@@ -25,7 +25,7 @@ usual place is
 In the root of the tree, run
 .BR ./INSTALL .
 This script builds the Plan 9 build program
-.IR mk (1)
+.IM mk (1)
 if necessary,
 cleans all previously built object files and libraries out of the tree,
 rebuilds and installs everything, and then cleans up.
@@ -109,7 +109,7 @@ can safely be repeated to rebuild the system from scratch.
 .PP
 Once the system is built for the first time,
 it can be maintained and rebuilt using
-.IR mk (1).
+.IM mk (1) .
 To rebuild individual commands or libraries,
 run
 .B mk
@@ -119,7 +119,7 @@ and
 .B clean
 in the appropriate source directory
 (see
-.IR src (1)).
+.IM src (1) ).
 .SH FILES
 .TP
 .B \*9/lib/moveplan9.files
@@ -132,7 +132,7 @@ the script that edits the files
 .TP
 .B \*9/src/mkmk.sh
 the shell script used to build
-.IR mk (1)
+.IM mk (1)
 .TP
 .B \*9/dist/manweb
 the shell script that builds the HTML manual
@@ -148,5 +148,5 @@ logged output from the last run of
 a summary of
 .B install.log
 .SH SEE ALSO
-.IR intro (1),
-.IR git (1)
+.IM intro (1) ,
+.IM git (1)