growfs script: fix config-dependent errors

• awk sometimes formatted swapbase as floating point, which gpart does not accept; force int.

• Fix typo in code for checking vm.max_swap_pages.

• While here, do not set kernel env if "gpart add" fails.

• Add "gpart show" before modification to verbose output.

Reported by: marklmi at yahoo dot com
Tested by: marklmi at yahoo dot com