Linux port

Linux port has been completed and verified on up-to-date Arch64 and Synology DSM 6.x - synology_braswell_916+: resulting INPX files are identical to Windows builds. Linux binaries are statically built, so they should run on wide variety of Intel 64 bits Linux-es.

Code changed some - lib2inpx lost "follow-symlinks" and gained "out-dir" parameters. Return code from libget2 and libmerge changed to work for Posix shells. There is a new bash script fb2_flibusta.sh (which basically does the same thing as fb2_flibusta.ps1).