bus-socket: Fix line_begins() to accept word matching full string
    
    The switch to memory_startswith() changed the logic to only look for a space or
    NUL byte after the matched word, but matching the full size should also be
    acceptable.
    
    This changed the behavior of parsing of "AUTH\r\n", where m will be set to 4,
    since even though the word will match, the check for it being followed by ' '
    or NUL will make line_begins() return false.
    
    Tested:
    
    - Using netcat to connect to the private socket directly:
      $ echo -ne '\0AUTH\r\n' | sudo nc -U /run/systemd/private
      REJECTED EXTERNAL ANONYMOUS
    
    - Running the Ignition blackbox test:
      $ sudo sh -c 'PATH=$PWD/bin/amd64:$PATH ./tests.test'
      PASS
    
    Fixes: d27b725abf64a19a6b2f99332b663f17ad046771
    (cherry picked from commit 3f10c66270b74530339b3f466c43874bb40c210f)
    
    Resolves: #1692991
    
    patch_name: 0106-bus-socket-Fix-line_begins-to-accept-word-matching-f.patch
    present_in_specfile: true
    location_in_specfile: 106
    squash_commits: true