libc: Add definitions for strnlen, strndup and strlcpy to string.h

libc/include/string.h

@@ -26,6 +26,9 @@ extern "C"
     /* Return the length of a null-terminated string. */
     size_t strlen(const char* str);
 
+    /* Return the length of a fixed-size null-terminated string. */
+    size_t strnlen(const char* str, size_t max);
+
     /* Compare two null-terminated strings. */
     int strcmp(const char* a, const char* b);
 
@@ -56,6 +59,12 @@ extern "C"
     /* Separate a string into several tokens. */
     char* strtok(char* str, const char* delim);
 
+    /* Return a heap-allocated copy of a fixed-size string. */
+    char* strndup(const char* str, size_t max);
+
+    /* Copy up to max bytes of src into dest, adding a null terminator at the end. */
+    size_t strlcpy(char* dest, const char* src, size_t max);
+
 #ifdef __cplusplus
 }
 #endif