To be honest, I had a little misunderstanding of the fallocate(1)-
utility.
It turns out that it only adds preallocated space if the length
exceeds the size of an already existing file.
No truncation happens.
Thus, I fixed the wording accordingly.
1) Simplify the manpage. Just refer to fallocate(2) and stop trying
to list supported file systems. This can change and everbody
with common sense can bring up the relevant manpages of a given
operating system himself.
Use the num-semantics.
2) Use estrtonum() instead of estrtol().
3) Allow multiple arguments.