Fix exec of script when script launched by name

[nchaimov]

Fixes a bug in Spindle found in the tau-cuda test of the E4S testsuite. When a script is executed, the kernel implementation replaces the original argv[0] with a string that is the absolute path to the script if argv[0] was not already a path. Thus, a case like:

export PATH="$PWD/interpreter_test_dir:$PATH"
interpreter_test_perl

where interpreter_test_perl is in interpreter_test_dir would fail with Spindle because the argument to the interpreter would be interpreter_test_perl rather than the path, so that the interpreter could not find the input script.

Fixed by mimicking kernel behavior: if already a path, use original argv[0] as is; otherwise substitute with the absolute path.

Also adds a test which fails without this fix:

Running: ./run_driver --serial ./interpreter_test
Can't open perl script "interpreter_test_perl": No such file or directory
FAILED.
SOME TESTS FAILED

and which passes with the fix:

Running: ./run_driver --serial ./interpreter_test
PASSED.
ALL TESTS PASSED

I have rerun the E4S testsuite and verified that this both fixes the tau-cuda test failure and does not introduce any new failures.