libc+libos: Properly propagate errors through fgetc() and File::getchar()

This restores proper ^C behavior in the shell.

libc/src/stdio.cpp

@@ -432,7 +432,12 @@ extern "C"
     {
         u8 value;
         ssize_t rc = read_from_buffer(stream, &value, 1);
-        if (rc <= 0) return EOF;
+        if (rc < 0)
+        {
+            stream->_err = 1;
+            return EOF;
+        }
+        else if (rc == 0) { return EOF; }
         return value;
     }
 

libos/src/File.cpp

@@ -149,9 +149,9 @@ namespace os
         int current;
         while (true)
         {
-            current = fgetc(m_file);
+            current = TRY(getchar());
 
-            if (current == -1) break;
+            if (current == EOF) break;
 
             TRY(data.try_append((char)current));
 
@@ -208,7 +208,9 @@ namespace os
 
     Result<int> File::getchar()
     {
-        return fgetc(m_file);
+        int rc = fgetc(m_file);
+        if (rc == EOF && ferror(m_file)) return err(errno);
+        return rc;
     }
 
     void File::set_close_on_exec()