libc: Some enhancements to syslog(3)

This is a re-application of commit
2d82b47a5b4ef18550565dd55628d51f54d0af2e, which was reverted since it
broke with syslog daemons that don't adjust the /dev/log recv buffer
size. Now that the default is large enough to accomodate 8KB messages,
restore support for large messages.

PR: 260126