don't bother clamping bytes_to_write to system_bufsize.
- prefer sendfile over mmap
- remove complicated failed-optimization when we have mmap which tries to
  Combine headers and initial part of response in first packet. May
  Re-introduce something like that later but for now this is too
  Complicated, fragile, and error-prone.
- don't clamp bytes_to_write to system_bufsize
- change simple_itoa prototype to take uint64_t
- use time_t in get_commonlog_time for temporary variable
  instead of unsigned int
- use socklen_t instead of int for setsockopt arg 3
- clean up comments in simple_itoa
- #include compat.h at the top before other includes, there is no need
  to #include config.h as compat.h does that for us.
add a very small comment at the end of defines.h
don't accidentally use global variable when we mean to use similarly
named local variable. also use sizeof(X) instead of (legal) sizeof X.
sizeof behaves like a function it should look like one.
- move of #defines and #ifdefs which are normally disabled to defines.h
- make internal access variables use unsigned int instead of int
use TESTING not TEST as the #define to check for
use sizeof(some fixed-size char buf) vs. SOME_MACRO
hide socketbufsize in #ifdef
- make mmap support optional