cache: avoid hardcoded CACHE_PATH_CUTOFF
Compute the cutoff at compile time instead which will avoid the need
for a CHERI case.
Correct the comment about CACHE_PATH_CUTOFF. It dates to 5.2.0 not 4.4
BSD. Historic values are:
32 - introduced (c2935410f6d5f)
35 - NUL terminated and bumped (5d5c174869104)
39 - sized to alignment (bb48255cf5eff)
45 - bumped to improve efficency on 64-bit (3862838921eb8)
No functional change.
Suggested by: jhb
Effort: CHERI upstreaming
Sponsored by: DARPA, AFRL