Fix markup where C include files get named in <...> form; bug reported by

Lorenzo M. Catucci <lorenzo@argon.roma2.infn.it>.

Doc/lib/libgrp.tex

@@ -9,7 +9,7 @@ This module provides access to the \UNIX{} group database.
 It is available on all \UNIX{} versions.
 
 Group database entries are reported as 4-tuples containing the
-following items from the group database (see \file{<grp.h>}), in order:
+following items from the group database (see \code{<grp.h>}), in order:
 \code{gr_name},
 \code{gr_passwd},
 \code{gr_gid},

Doc/lib/libpwd.tex

@@ -9,7 +9,7 @@ This module provides access to the \UNIX{} password database.
 It is available on all \UNIX{} versions.
 
 Password database entries are reported as 7-tuples containing the
-following items from the password database (see \file{<pwd.h>}), in order:
+following items from the password database (see \code{<pwd.h>}), in order:
 \code{pw_name},
 \code{pw_passwd},
 \code{pw_uid},

Doc/lib/libsignal.tex

@@ -82,7 +82,7 @@ The variables defined in the \module{signal} module are:
   All the signal numbers are defined symbolically.  For example, the
   hangup signal is defined as \constant{signal.SIGHUP}; the variable names
   are identical to the names used in C programs, as found in
-  \file{<signal.h>}.
+  \code{<signal.h>}.
   The \UNIX{} man page for `\cfunction{signal()}' lists the existing
   signals (on some systems this is \manpage{signal}{2}, on others the
   list is in \manpage{signal}{7}).

Doc/lib/libstdwin.tex

@@ -425,8 +425,7 @@ Suitable names include
 \code{'watch'}
 and
 \code{'plus'}.
-On X11, there are many more (see
-\file{<X11/cursorfont.h>}).
+On X11, there are many more (see \code{<X11/cursorfont.h>}).
 \end{sloppypar}
 \end{funcdesc}