Add libc_vfs plugin and test

Issue #999
Retire libc_log, use libc_vfs instead (WiP)
fixup: retire libc_log
fixup: retire libc_log (mupdf)
fixup: retire libc_log
fixup: retire libc_log
fixup: retire libc_log (lighttpd)
fixup: retire libc_log
fixup: retire libc_log (python)
fixup: retire libc_log
fixup: retire libc_log
fixup: retire libc_log (libc_fs_tar_fs)
fixup: retire libc_log (test/libc_block)
fixup: retire libc_log (rump_ext2, rump_fat)
fixup: retire libc_log (test/lx_rmap)
fixup: retire libc_log (file_terminal)
fixup: retire libc_log (qt5 apps)
fixup: retire libc_log (libc_ffat_fs)
fixup: retire libc_log (arora, eglgears, usb_net)
fixup: retire libc_log (avplay)