Update s-maxage separately from max-age for CACHE_ALL_STATIC client_ttl
Update s-maxage separately from max-age for CACHE_ALL_STATIC client_ttl

As part of the change, we refactor gfe::header_utils::GetMaxAge() into another
GetClientMaxAge() version that doesn't look at s_maxage, so we can use that
to properly clamp just the client portion with client_ttl_secs.

Also, fix an odd little glitch that max-age=-1 (and s-maxage=-1) was
correctly identified by the code as illegal, but not max-age=-2 or smaller.

Protected by gfe2_reloadable_flag_clamp_smaxage.

PiperOrigin-RevId: 339670897
Change-Id: Ice213d4ba503d92fb077ebd25ee0dc84136c0dda

google/quiche,google/quiche,google/quiche,google/quiche