fix compression pointer for big (>255) DNS responses

compatibility with glibc 2.3, warn_unused_result attribute for write()

always use buffer, if connection is buffered,
this fixes OpenSSL "bad write retry" error, when
*) nginx passed a single buf greater than our buffer (say 32K) to OpenSSL,
*) OpenSSL returns SSL_ERROR_WANT_WRITE,
*) after some time nginx has to send a new data,
*) so there are at least two bufs nginx does pass them directly to OpenSSL,
*) but copies the first buf part to buffer, and sends the buffer to OpenSSL.
*) because the data length is lesser than it was in previous SSL_write():
   16K < 32K, OpenSSL returns SSL_R_BAD_WRITE_RETRY.

strip charset name quotes

fix divide by zero if max_fails=0

fix $r->header_in() for "User-Agent", "Connection", and "Host"
broken in r2091 and r2093 merges

leave HEAD method while error_page redirection

*) allow file existence test for POST requests in static module
*) do not discard body while error_page redirection

ignore NGX_EACCES error while directory test

disable error_page loop in 500 error when recursive errors are enabled