seq_buf: Add seq_buf_init_append()

Several strlcat() call sites being converted to seq_buf need behavior
seq_buf doesn't currently provide. The normal seq_buf_init() always
empties the buffer (sets its "len" to 0) via seq_buf_clear(). Code
migrating from strlcat(buf, ...), which appends to whatever buf already
contains, can't use seq_buf_init() without discarding that existing
content. Add seq_buf_init_append(), which preserves the existing
contents and positions the seq_buf to append after it. A buffer with
no NUL within its size starts out overflowed, as strlcat() treats it
as already truncated. Add KUnit tests for behavior coverage.

Tests passed under qemu on ARCH=x86_64 with GCC 16.2.0 and CONFIG_KASAN=y,
and on big-endian ARCH=s390 with GCC s390x-linux-gnu 16.2.0.

Assisted-by: LLM
Signed-off-by: Bill Wendling <morbo@google.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Co-developed-by: Kees Cook <kees@kernel.org>
Link: https://patch.msgid.link/20261005155708.1471260-9-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>

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