MINOR: compression: pass the content type to the algorithms' init function

MINOR: compression: pass the content type to the algorithms' init function

Now that we have slz 1.4-dev4 with support for text-aware compression
using an alternate huffman tree, let's implement support for enabling it.
In order to permit this, the algorithms' init() function now takes a new
argument, <ctype>, which is either COMP_CT_GENERIC (0) for unknown or
binary contents, or COMP_CT_TEXT (1) for text contents.

comp_prepare_compress_request() and select_compression_response_header()
set it to COMP_CT_TEXT when the Content-Type header starts with "text/",
regardless of the "compression type" lists. The config-time checks of the
algorithms use COMP_CT_GENERIC.

The slz algorithms select SLZ_CT_TEXT after their init for text contents.
libslz has no dedicated macro for the content types, but they were
introduced right after SLZ_MAX_OVERHEAD(), which is thus used to detect an
external libslz that supports them. zlib and identity ignore the argument.

On an 81 kB HTML file, the response is 22.0 kB when served as text/html
instead of 25.4-27.1 kB when served as application/octet-stream at level 1
(the size depends on how the server sends the data).