Make \tl_if_in: conditionals alignment-safe

This is simply a matter of surrounding the code with \if_false: { \fi:
and \if_false: } \fi: .  I've also moved the \group_align_safe_begin/end:
used in \tl_replace functions to a more natural place (and explained).


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5323 de43f980-851b-0410-b2f7-c40aca1f87e0