[ng] dist: do not exceed command line length limits, even with many files

[ng] dist: do not exceed command line length limits, even with many files

* lib/am/distdir.am (am.mkdir-for-dist, am.write-list-of-distfiles): New
internal functions.
(distdir): Using them and the 'am.xargs-map' function, in a way that
should prevent failures due to excessive command line lengths.

With this, the tests 'dist-many.sh' and 'dist-many2.sh' pass on the
systems where we verified they were previously failing.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>