Add \file_if_exist_input:n(TF) as experimental functions

There is also a fix in the test file: worng name used in tests!


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5170 de43f980-851b-0410-b2f7-c40aca1f87e0
Various improvements to \tl_(g)set_from_file:Nn (closes #177)

Most of these are taken directly from suggestiosn by Qing Lee. I've used a
slightly 'cleaner' approach for the expansion-out-of-a-group chain. The
one place I've not changed is where \tl_set:Nx is used in storing the grabbed
file. This is needed for the same reason we use \edef internally for \tl_set:Nn:
it allows for `#` tokens.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5171 de43f980-851b-0410-b2f7-c40aca1f87e0
Update test files for name changes in \tl_(g)set_from_file:Nn


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5172 de43f980-851b-0410-b2f7-c40aca1f87e0