lime-utils: init the package with the script unique_append

[a-gave]

[WIP] still untested

lime-system: utils.lua use nixio.fs for file operations
This includes also a similar minor fix:

• fix: use the lua native #var to count tableLength(var)

lime-utils: init the package with the script unique_append
The following changes are made:

• initialize the package lime-utils with the script unique_append
• add a dependency on lime-utils to all packages
• always 'exit 0' on uci defaults that add new crontabs, to execute them
  only at firstboot and not at every boot

Fixes: #949

[ilario]

I like the new implementation!
But this function passed to be true also for directories to be true only for regular files, and this requires updating this test:

lime-packages/tests/test_test_utils.lua

Line 55 in 1e0e15e

assert.is_true(utils.file_exists(dir))

and some (mostly unused) code here (just checked the ones where the variable name looked like a directory's one):

lime-packages/packages/lime-system/files/usr/lib/lua/lime/generic_config.lua

Line 84 in e2876af

if not utils.file_exists(dst_dirname) then

lime-packages/packages/eupgrade/files/usr/lib/lua/eupgrade.lua

Line 16 in e2876af

if not utils.file_exists(workdir) then

lime-packages/packages/pirania/files/usr/lib/lua/read_for_access/read_for_access.lua

Line 12 in e2876af

if not utils.file_exists(workdir) then

And also here as it seems that keeping directories over updates is supported (according to lime-example.txt, I did not check):

lime-packages/packages/lime-system/files/usr/lib/lua/lime/utils.lua

Line 437 in e2876af

if utils.file_exists(list) then

[ilario]

I love the new lime-utils package!!!
Do you think that eventually, slowly slowly, it could contain the utils.lua, config.lua and modules.lua files from lime-system, as proposed in #771 ?

[ilario]

Do you think this package could be used out of LibreMesh and be pushed to OpenWrt's repositories, eventually?
In this case, you could already place the package in the Utilities category.