BUFSIZ-related pessimization in fvwrite.c

[enh <enh%google.com@localhost>]

(if anyone knows the equivalent freebsd list, please add them --- this
code is the same between all three bsds.)

Apple patched their copy of the FreeBSD fvwrite.c to change BUFSIZ to
INT_MAX to massively improve the performance of large unbuffered
writes[1], and patched the buffered case to use the largest multiple
of the buffer size to massively improve the performance of large
buffered writes:

https://github.com/apple-oss-distributions/Libc/commit/c5a3293354e22262702a3add5b2dfc9bb0b93b85#diff-3b844a19cfb0aab1a23f7fbc457d3bce7453513730c489a72f66ff89d6557ff3

i've tested similar changes to openbsd's fvwrite.c with great results.

is it a _requirement_ that buffered writes _only_ happen in multiples
of the buffer size? that seems unlikely to me (a) because of short
writes and (b) musl just does a writev() of what's in the buffer and
what it was just given in case of a larger-than-buffer write and (c)
we already changed the openbsd fread() to read directly into the
caller's buffer regardless of size, so the equivalent behavior on the
_write_ side seems reasonable to me?

____
1. BUFSIZ is only 1024 bytes on the three BSDs and Android/iOS. glibc uses 8KiB.