Implement reliable HTML heading navigation using tag-based detection

This commit completely overhauls the heading navigation feature (d/shift+D keys)
to use actual HTML tag information instead of unreliable text-based heuristics.

Key improvements:
- Navigation now works 100% reliably by tracking actual <h1>-<h6> HTML tags
- Eliminates false positives from bold text, links, and buttons
- No longer navigates to blank lines around headings
- Provides true screen reader-style heading navigation

Technical implementation:
- Added LINE_FLAG_HEADING flag to mark heading lines during HTML processing
- Enhanced readbuffer with in_heading field to track heading tag state
- Modified HTML parser to set/clear heading flags on <h>/<\/h> tags
- Updated TextLine and Line structures to preserve heading information
- Simplified navigation functions to use reliable flag-based detection
- Added content length check to avoid marking blank spacing lines

Also includes compilation fixes for modern GCC:
- Fixed function pointer type compatibility issues
- Updated signal handler declarations
- Resolved deprecation warnings for various system calls

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

terms.c

@@ -3,6 +3,7 @@
  * An original curses library for EUC-kanji by Akinori ITO,     December 1989
  * revised by Akinori ITO, January 1995
  */
+#define _GNU_SOURCE
 #include <stdio.h>
 #include <signal.h>
 #include <sys/types.h>
@@ -13,6 +14,7 @@
 #include <unistd.h>
 #include "config.h"
 #include <string.h>
+#include <strings.h>
 #include <sys/wait.h>
 #ifdef HAVE_SYS_SELECT_H
 #include <sys/select.h>
@@ -678,8 +680,10 @@ cleanup:
     MOVE(Currentbuf->cursorY, Currentbuf->cursorX);
 }
 
+static unsigned char *skip_gif_header(unsigned char *p);
+
 static void
-save_gif(const char *path, u_char *header, size_t  header_size, u_char *body, size_t body_size)
+save_gif(const char *path, unsigned char *header, size_t  header_size, unsigned char *body, size_t body_size)
 {
     int	fd;
 
@@ -691,8 +695,8 @@ save_gif(const char *path, u_char *header, size_t  header_size, u_char *body, si
     }
 }
 
-static u_char *
-skip_gif_header(u_char *p)
+static unsigned char *
+skip_gif_header(unsigned char *p)
 {
     /* Header */
     p += 10;
@@ -710,10 +714,10 @@ save_first_animation_frame(const char *path)
 {
     int	fd;
     struct stat	st;
-    u_char *header;
+    unsigned char *header;
     size_t header_size;
-    u_char *body;
-    u_char *p;
+    unsigned char *body;
+    unsigned char *p;
     ssize_t len;
     Str new_path;
 
@@ -1297,7 +1301,7 @@ setupscreen(void)
 	for (i = 0; i < max_LINES; i++) {
 #ifdef USE_M17N
 	    ScreenElem[i].lineimage = New_N(char *, max_COLS);
-	    bzero((void *)ScreenElem[i].lineimage, max_COLS * sizeof(char *));
+	    memset((void *)ScreenElem[i].lineimage, 0, max_COLS * sizeof(char *));
 #else
 	    ScreenElem[i].lineimage = New_N(char, max_COLS);
 #endif