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/man7/ms.7

@@ -18,7 +18,7 @@ ms \- macros for formatting manuscripts
 This package of
 .I nroff
 and
-.IR troff (1)
+.IM troff (1)
 macro definitions provides a canned formatting
 facility for tech%nical papers in various formats.
 .PP
@@ -38,11 +38,11 @@ impunity after the first
 .LR .na .
 .PP
 Output of the
-.IR eqn (1),
-.IR tbl (1),
-.IR pic (1)
+.IM eqn (1) ,
+.IM tbl (1) ,
+.IM pic (1)
 and
-.IR grap (1)
+.IM grap (1)
 preprocessors
 for equations, tables, pictures, and graphs is acceptable as input.
 .SH FILES
@@ -57,8 +57,8 @@ Tenth Edition, Volume 2.
 .br
 .IR eqn (1), 
 .IR troff (1), 
-.IR tbl (1),
-.IR pic (1)
+.IM tbl (1) ,
+.IM pic (1)
 .SH REQUESTS
 .ta \w'..ND \fIdate\fR 'u +\w'Initial 'u +\w'Cause 'u
 .br
@@ -153,7 +153,7 @@ Implies
 produced by
 .I neqn
 or
-.IR eqn (1).
+.IM eqn (1) .
 .ti0
 \fL\&.EQ\fP \fIx y\fR	-	yes	Display equation.
 Equation number is
@@ -234,7 +234,7 @@ is subsection level (default 1).
 \fL\&.P2\fP	-	yes	End program display.
 .ti0
 \fL\&.PE\fP	-	yes	End picture; see
-.IR pic (1).
+.IM pic (1) .
 .ti0
 \fL\&.PF\fP	-	yes	End picture; restore vertical
 position.
@@ -280,7 +280,7 @@ font automatically bold.
 Default is 5 10 15 ...
 .ti0
 \fL\&.TE\fP	-	yes	End table; see
-.IR tbl (1).
+.IM tbl (1) .
 .ti0
 \fL\&.TH\fP	-	yes	End heading section of table.
 .ti0