Use buffered stdio

Our generation of HTML triggers many small write(2) syscalls
which is inefficient.

Time output on a horrible query against my git.git mirror
shows significant performance improvement:

QUERY_STRING='id=2b93bfac0f5bcabbf60f174f4e7bfa9e318e64d5&id2=d6da71a9d16b8cf27f9d8f90692d3625c849cbc8'
PATH_INFO=/mirrors/git.git/diff
export QUERY_STRING PATH_INFO
time ./cgit >/dev/null

Before:
real    0m1.585s
user    0m0.904s
sys     0m0.658s

After:
real    0m0.750s
user    0m0.666s
sys     0m0.076s

1 files changed, 1 insertions, 1 deletions

diff --git a/cgit.c b/cgit.c

index 2efa962..3a65885 100644
--- a/cgit.c
+++ b/cgit.c
@@ -683,7 +683,7 @@ static inline void authenticate_post(void)
 		len = MAX_AUTHENTICATION_POST_BYTES;
 	if ((len = read(STDIN_FILENO, buffer, len)) < 0)
 		die_errno("Could not read POST from stdin");
-	if (write(STDOUT_FILENO, buffer, len) < 0)
+	if (fwrite(buffer, 1, len, stdout) < len)
 		die_errno("Could not write POST to stdout");
 	cgit_close_filter(ctx.cfg.auth_filter);
 	exit(0);