tmac/tmac.an*: work around formatter bug

...when rendering some man pages, such as those of ncurses.

I did not manage (nor seriously attempt) to identify the root cause of
this bug.  ncurses's use of `SH` and `SS` man(7) macros is
unremarkable.[1]  I cannot account for why the less(1) man page renders
fine and ncurses pages like insstr(3) do not.  But render badly they do,
emitting *roff logic as formatted output.

```
$ 9 nroff -man $(man -w insstr) | grep -F .ss | cat -v
     "'''if^GNAME^GSYNOPSIS^G .ss 18 NAME
     "'''if^GSYNOPSIS^GSYNOPSIS^G .ss 18 SYNOPSIS
     "'''if^GDESCRIPTION^GSYNOPSIS^G .ss 18 DESCRIPTION
     "'''if^GRETURN^GSYNOPSIS^G .ss 18 RETURN VALUE
     "'''if^GNOTES^GSYNOPSIS^G .ss 18 NOTES
     "'''if^GEXTENSIONS^GSYNOPSIS^G .ss 18 EXTENSIONS
     "'''if^GPORTABILITY^GSYNOPSIS^G .ss 18 PORTABILITY
     "'''if^GHISTORY^GSYNOPSIS^G .ss 18 HISTORY
     "'''if^GSEE^GSYNOPSIS^G .ss 18 SEE ALSO
```

With this patch:

```
$ 9 nroff -man $(man -w insstr) | grep -F .ss | cat -v | grep . || echo NO OUTPUT
NO OUTPUT
```

I do observe that the problem seems to correspond to the only use in the
package of the old-fashioned `'''` commenting convention _within a macro
definition_.  I have a notion of how GNU troff works, but little about
AT&T troff.  That said, if I were to try to get to the bottom of this
problem, I'd look into if and how the no-break command character is
handled differently in copy mode.  I see nothing in CSTR #54 to suggest
that the command characters have different meanings in copy mode and its
complement.[2]

My solution is to use idiomatic comment syntax inside macro definitions.

[1] d5dc8a4a7c/man/curs_insstr.3x (L47)

[2] unnamed in AT&T documentation but which I term "interpretation mode"
    in groff

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