feat: add string manipulation functions derived from musl libc

This commit introduces a variety of string manipulation functions, including:
- `memmem`: Searches for a substring in a memory area.
- `memmove`: Safely copies memory areas, handling overlaps.
- `mempcpy`: Copies memory and returns a pointer to the end of the destination.
- `memrchr`: Finds the last occurrence of a character in a memory area.
- `memset`: Fills a memory area with a specified value.
- `rindex`: Returns the last occurrence of a character in a string.
- `stpcpy` and `stpncpy`: Copies strings and returns a pointer to the end.
- `strcat`, `strchr`, `strcspn`, `strlcat`, `strlcpy`, `strlen`, `strncat`, `strncmp`, `strncpy`, `strnlen`, `strpbrk`, `strrchr`, `strstr`, `strtok`, and `strtok_r`: Various string manipulation functions for concatenation, searching, and tokenization.

Assembly implementations for `memcpy`, `memmove`, and `memset` are also included for optimized performance on x86_64 architectures.

1 files changed, 13 insertions, 0 deletions

diff --git a/na/lib/string/strncat.c b/na/lib/string/strncat.c
new file mode 100644
index 0000000..a144e37
--- /dev/null
+++ b/na/lib/string/strncat.c
@@ -0,0 +1,13 @@
+/* SPDX-License-Identifier: MIT */
+/* Derived from musl libc (Copyright © 2005-2020 Rich Felker, et al.) */
+
+#include <string.h>
+
+char *strncat(char *restrict d, const char *restrict s, size_t n)
+{
+	char *a = d;
+	d += strlen(d);
+	while (n && *s) n--, *d++ = *s++;
+	*d++ = 0;
+	return a;
+}