Correct some sizeof(size_t) != sizeof(unsigned long) typing errors

Correct some sizeof(size_t) != sizeof(unsigned long) typing errors

Fix size_t vs. unsigned long pointer mismatch warnings introduced
with the addition of strbuf_detach().

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>