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/man3/mach-map.3

@@ -133,10 +133,10 @@ via
 data structures that provides access to an address space
 and register set.
 The functions described in
-.IR mach-file (3)
+.IM mach-file (3)
 are typically used to construct these maps.
 Related library functions described in 
-.IR mach-symbol (3)
+.IM mach-symbol (3)
 provide similar access to symbol tables.
 .PP
 Each
@@ -178,7 +178,7 @@ The
 .B rw
 function is most commonly used to provide
 access to executing processes via
-.IR ptrace (2)
+.IM ptrace (2)
 and to zeroed segments.
 .PP
 .I Allocmap
@@ -346,7 +346,7 @@ such locations are useful for passing specific constants to
 functions expect locations, such as
 .I unwind
 (see
-.IR mach-stack (3)).
+.IM mach-stack (3) ).
 .PP
 .I Loccmp
 compares two locations, returning negative, zero, or positive
@@ -360,7 +360,7 @@ which are ordered before indirections.
 .PP
 .I Locfmt
 is a 
-.IR print (3)-verb
+.IM print (3) -verb
 that formats a
 .B Loc
 structure
@@ -371,7 +371,7 @@ Indirection locations are needed in some contexts (e.g., when
 using
 .I findlsym
 (see
-.IR mach-symbol (3))),
+.IM mach-symbol (3) )),
 but bothersome in most.
 .I Locsimplify
 rewrites indirections as absolute memory addresses, by evaluating
@@ -397,8 +397,8 @@ function families as necessary.
 .SH SOURCE
 .B \*9/src/libmach
 .SH "SEE ALSO"
-.IR mach (3),
-.IR mach-file (3)
+.IM mach (3) ,
+.IM mach-file (3)
 .SH DIAGNOSTICS
 These routines set
 .IR errstr .