Trim whitespace inside <arg> elements

It seems that some versions of Python add newlines here:

http://thread.gmane.org/gmane.comp.file-systems.zero-install.devel/7074
Disable multiarch support on Linux if /lib/ld-linux.so.2 is missing

Otherwise, we try to run 32-bit binaries and get a useless error
message.
Fix some more OCaml-4-only issues
Emulate Filename.get_temp_dir_name to support OCaml 3.12
Release 2.3.3
Start development series 2.3.3-post
Merge branch 'b2.3' with fixes from 2.3.3