prefill: remove depedency on lua-filesystem (lfs)

Package lua-filesystem (library lfs) in version for Lua 5.1 is not available in RHEL 8, and we in fact need only one little function from it. Let's get rid of the dependency.

Proposed approach:

  • add small C function to get value equivalent to lfs.attributes(filenamename).modification to our auxiliary library lib/utils.c
  • replace lfs library in modules/prefill.lua with call to ffi.C.our_new_func()
  • remove lua-filesystem references from packaging files
Edited by Lukas Jezek
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