printbuf: preserve self-append sources across growth

When the appended data points into the printbuf itself, growing the buffer may invalidate the source pointer before it is copied. Preserve the source offset across realloc and use memmove for internal overlap.

tests/test_printbuf.expected

@@ -21,6 +21,9 @@ Append to just after resize: 32, [XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX]
 Buffer size after printbuf_strappend(): 16, [XXXXXXXXXXXXXXXX]
 test_printbuf_memappend: end test
 ========================================
+test_printbuf_self_append: starting test
+test_printbuf_self_append: end test
+========================================
 test_sprintbuf: starting test
 Buffer length: 0
 sprintbuf to just after resize(31+1): 32, [XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX], strlen(buf)=32