http: use a larger buffer for ->getline responses

http: use a larger buffer for ->getline responses

64K matches the Linux pipe default, and matches what we use in
httpd/async and qspawn.  This should reduce syscalls used for
serving git packs via dumb HTTP and any ->getline code paths
used by other PSGI code.

This appears to speed up HTML rendering by w3m when serving
giant HTML responsees from the Devel::Mwrap::PSGI memory
debugger.