Use protobuf’s Gzip{Input,Output}Stream wrapper around zlib

This removes our direct zlib dependency (although of course protobuf
still uses it internally), removes a fixed 4 MiB buffer and its
corresponding limit on the terminal size, reduces some string copying,
and deletes some code.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

(Closes #230.)

configure.ac

@@ -155,8 +155,6 @@ AS_IF([test x"$with_utempter" != xno],
       [AC_MSG_WARN([Unable to find libutempter; utmp entries will not be made.])],
       [AC_MSG_ERROR([--with-utempter was given but libutempter was not found.])])])])
 
-AC_SEARCH_LIBS([compress], [z], , [AC_MSG_ERROR([Unable to find zlib.])])
-
 AC_ARG_WITH([skalibs],
   [AS_HELP_STRING([--with-skalibs[=DIR]],
      [root directory of skalibs installation])],