xsession: Make xsession script testable.

A few minor related changes are included. E.g. we actually want to pass
along blank lines, so remove that check (which didn't work anyways as \n
was still present).

We replace that with a simple filter in the template installation
process to allow there to be source lines in the base template that
don't get passed to the installed result. This allows for shell script
that prevents running the full command suite in the base without
affecting the installed script.

It is required to be able to actually run the base script as the "set
-u" check for unset variables doesn't work when "set -n" (syntax check)
is in use. These set commands can be passed directly to /bin/sh to have
them in effect when the script starts (and are both part of POSIX and
seem to work even with busybox sh).

Additionally, the path_add function needs to actually check for unset
variables now. Luckily there is existing Bourne shell syntax for that.

The observant reviewer will note that the testsuite does not currently
pass because there is a mispeled lib_suffix in the base, that will be
fixed in the next commit.