Switch recommendation from 2 spaces to 4 spaces.

My motivations:

1. 4 spaces is more common. People using 2 spaces for indentation
   generally nest too deep, using too few functions.

2. Currently, 4 spaces is *already* the norm. See this:

        $ for x in `find src -name '*.c' -or -name '*.cpp'`; do \
          if grep -q '^      [^ ]' $x; then y='two space'
          elif grep -q "`printf '\t\t'`" $x; then y='tab'
          else y='regular 4 space'; fi; \
          printf '%-32s%s indent\n' "$x" "$y"; done | sort -k2
        src/comp.c                      regular 4 space indent
        src/sslinit.c                   regular 4 space indent
        src/infile.cpp                  regular 4 space indent
        src/milenage.c                  regular 4 space indent
        src/rijndael.c                  regular 4 space indent
        src/screen.cpp                  regular 4 space indent
        src/fortune.cpp                 regular 4 space indent
        src/message.cpp                 regular 4 space indent
        src/strings.cpp                 regular 4 space indent
        src/xp_parser.c                 regular 4 space indent
        src/deadcall.cpp                regular 4 space indent
        src/listener.cpp                regular 4 space indent
        src/variables.cpp               regular 4 space indent
        src/prepare_pcap.c              regular 4 space indent
        src/reporttask.cpp              regular 4 space indent
        src/send_packets.c              regular 4 space indent
        src/sip_parser.cpp              regular 4 space indent
        src/socketowner.cpp             regular 4 space indent
        src/sslthreadsafe.c             regular 4 space indent
        src/sipp_unittest.cpp           regular 4 space indent
        src/call_generation_task.cpp    regular 4 space indent
        src/md5.c                       tab indent
        src/auth.cpp                    tab indent
        src/call.cpp                    two space indent
        src/sipp.cpp                    two space indent
        src/stat.cpp                    two space indent
        src/task.cpp                    two space indent
        src/time.cpp                    two space indent
        src/logger.cpp                  two space indent
        src/socket.cpp                  two space indent
        src/actions.cpp                 two space indent
        src/scenario.cpp                two space indent
        src/watchdog.cpp                two space indent
        src/rtpstream.cpp               two space indent