use buffered stdio (lightly tested)

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 2f07e6d..55ba735 100644
--- a/cgit.c
+++ b/cgit.c
@@ -667,7 +667,7 @@ 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);