Remove the gawk dependency.

This reverts commit 1814251453c8140f50170ad29d9105c1273d7e08.

Demote the gawk call back to awk and ensure that stderr is attached. GNU gawk
tolerates a missing stderr handle, but many utilities do not, which could be
why a regular awk call was unexplainably failing on some systems.

Use argv[0] instead of sh_path for consistency internally and with other Linux
drivers.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>