filter: add fprintf_filter function

This stops the code in cgit.c::print_repo needing to inspect the
cgit_filter structure, meaning that we can abstract out different filter
types that will have different fields that need to be printed.

Signed-off-by: John Keeping <john@keeping.me.uk>
filter: add interface layer

Change the existing cgit_{open,close,fprintf}_filter functions to
delegate to filter-specific implementations accessed via function
pointers on the cgit_filter object.

We treat the "exec" filter type slightly specially here by putting its
structure definition in the header file and providing an "init" function
to set up the function pointers.  This is required so that the
ui-snapshot.c code that applies a compression filter can continue to use
the filter interface to do so.

Signed-off-by: John Keeping <john@keeping.me.uk>
filter: introduce "filter type" prefix

This allows different filter implementations to be specified in the
configuration file.  Currently only "exec" is supported, but it may now
be specified either with or without the "exec:" prefix.

Signed-off-by: John Keeping <john@keeping.me.uk>
filter: allow for cleanup hook for filter types

At some point, we're going to want to do lazy deallocation of filters.
For example, if we implement lua, we'll want to load the lua runtime
once for each filter, even if that filter is called many times.
Similarly, for persistent exec filters, we'll want to load it once,
despite many open_filter and close_filter calls, and only reap the child
process at the end of the cgit process. For this reason, we add here a
cleanup function that is called at the end of cgit's main().

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
filter: basic write hooking infrastructure

Filters can now call hook_write and unhook_write if they want to
redirect writing to stdout to a different function. This saves us from
potential file descriptor pipes and other less efficient mechanisms.

We do this instead of replacing the call in html_raw because some places
stdlib's printf functions are used (ui-patch or within git itself),
which has its own internal buffering, which makes it difficult to
interlace our function calls. So, we dlsym libc's write and then
override it in the link stage.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
filter: add preliminary lua support

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>