__sfvwrite: allow writing buffers larger than BUFSIZ or st_blksize.

__sfvwrite: allow writing buffers larger than BUFSIZ or st_blksize.

This can vastly improve write performance.  For unbuffered writes,
we now write up to INT_MAX instead of BUFSIZE.  For buffered writes,
write the largest multiple of the buffer size (which may be st_blksize).
From Apple via enh at google.  OK tb@