ffi/loadlib: add module

[benoit-pierre]

Provide a ffi.loadlib helper, replacing ffi.load and util.ffiLoadCandidates for loading our libraries.

Prerequisite for #1920 & koreader/koreader#12545.

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This change is 

[NiLuJe]

Reviewed 23 of 23 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion

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ffi/jpeg.lua line 19 at r1 (raw file):

-- backward compatible: the major & patch numbers are always 0, and when
-- a new API version is made available, the minor number is incremented.
local turbojpeg = ffi.loadlib("turbojpeg", "0.3.0", "turbojpeg")

The idea behind the third argument is to fall-back to an unversioned library, right?

I wouldn't mind proper documentation about that in ffi/loadlib.lua itself ;).

(Specifically as far as this sort of stuff is concerned, as well as the util.loadSDL2 workflow where you can just pass a list of name, version tuples to a single call and they'll get chunked automatically).

[benoit-pierre]

Reviewed 23 of 23 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion

ffi/jpeg.lua line 19 at r1 (raw file):

-- backward compatible: the major & patch numbers are always 0, and when
-- a new API version is made available, the minor number is incremented.
local turbojpeg = ffi.loadlib("turbojpeg", "0.3.0", "turbojpeg")

The idea behind the third argument is to fall-back to an unversioned library, right?

Actually, if the goal is to maybe not ship our libraries, for projects we don't patch (e.g. to use system libraries when building Linux packages), then the code should be amended to ffi.loadlib("turbojpeg", "0.3.0") or ffi.load("turbojpeg"), so we do fallback to the unversioned system library if we're not shipping ours and the system version is different: if the version is too old, the symbols we need will be unresolved, otherwise, any newer version should be backward compatible.

I wouldn't mind proper documentation about that in ffi/loadlib.lua itself ;).

See #1939.