While a little-used feature, it doesn't seem to be necessary for the "underline font" to be global.

Also, its value is not introspectable.  Add a read-only register `.uf` for it.

This simple patch doesn't work because `get_underline_fontno` isn't in scope.

diff --git a/src/roff/troff/env.cpp b/src/roff/troff/env.cpp
index 5d844ed20..e12b9f34c 100644
--- a/src/roff/troff/env.cpp
+++ b/src/roff/troff/env.cpp
@@ -3539,6 +3539,7 @@ void init_env_requests()
   init_string_env_reg(".sty", get_style_name_string);
   init_string_env_reg(".tabs", get_tabs);
   init_int_env_reg(".u", get_fill);
+  init_int_env_reg(".uf", get_underline_fontno);
   init_vunits_env_reg(".v", get_vertical_spacing);
   init_hunits_env_reg(".w", get_prev_char_width);
   init_int_env_reg(".zoom", get_zoom);

Fixing this means moving some machinery from node.cpp to env.cpp.

We can see that this probably should have been the case all along.

src/roff/troff/env.cpp:    if (fontno == get_underline_fontno())
src/roff/troff/env.cpp:    if (prev_fontno == get_underline_fontno())
src/roff/troff/env.cpp:      curenv->fontno = curenv->pre_underline_fontno;
src/roff/troff/env.cpp:    curenv->pre_underline_fontno = curenv->fontno;
src/roff/troff/env.cpp:    curenv->fontno = get_underline_fontno();
src/roff/troff/env.cpp:      fontno = pre_underline_fontno;
src/roff/troff/env.cpp:    errprint("  font number before underlining: %1\n", pre_underline_fontno);
src/roff/troff/env.cpp:  init_int_env_reg(".uf", get_underline_fontno);
src/roff/troff/env.h:  int pre_underline_fontno;
src/roff/troff/node.cpp:static int underline_fontno = 2;
src/roff/troff/node.cpp:    underline_fontno = finfo.position;
src/roff/troff/node.cpp:int get_underline_fontno()
src/roff/troff/node.cpp:  return underline_fontno;
src/roff/troff/node.h:extern int get_underline_fontno();