Fix #896 - Stream decompress zipped files

[cemeyer]

Use the POSIX fopencookie(3) mechanism to produce a FILE object, and
then treat them the same as other non-mmapable streams (i.e., FIFOs).

Since some supported platforms do not support fopencookie(3) (Mac OS X,
maybe Cygwin, older BSDs), retain non-streaming zip file support.

Existing tests pass.

[cemeyer]

For example, with a large compressed zero corpus (70 MB compressed, 281 GB uncompressed), this makes completion actually possible:

Before patch:

$ ag -z foo test.out.xz
ERR: Unable to allocate 691011584 bytes to decompress file test.out.xz
ERR: Cannot decompress zipped file test.out.xz

After patch:

$ time ./ag -z foo test.out.xz
./ag -z foo test.out.xz  10.21s user 1.14s system 99% cpu 11.375 total

[ggreer]

Wow, very nice!