[ng] am.xargs-map: be usable with huge lists

[ng] am.xargs-map: be usable with huge lists

The previous implementation could eat all memory if the given list was
really huge -- but this function will be needed to operate on potentially
huge lists! (i.e., list of tests or files which can exceed the command-line
length limit of the system).

As a drawback, the function cannot recursively invoke itself anymore (not
even indirectly), but this is an acceptable limitation for its intended
uses.

* lib/am/header-vars.am (am.xargs-map): Reimplemented.
(am.max-cmdline-args): Redefine, as its semantics have changed.
(am.max-cmdline-args+1): Remove, no more needed.

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