Fix and improve swap_usage()
Fix and improve swap_usage()

- Fix a typo on awk syntax
- Use php functions to extract desired value from swapinfo output

NOYB/pfsense,dennypage/pfsense,ch1c4um/pfsense,pfsense/pfsense,phil-davis/pfsense,NewEraCracker/pfsense,ch1c4um/pfsense,BlackstarGroup/pfsense,ptorsten/pfsense,pfsense/pfsense,phil-davis/pfsense,jxmx/pfsense,NOYB/pfsense,ch1c4um/pfsense,BlackstarGroup/pfsense,pfsense/pfsense,pfsense/pfsense,jxmx/pfsense,NewEraCracker/pfsense,ch1c4um/pfsense,dennypage/pfsense,brunostein/pfsense,NewEraCracker/pfsense,ptorsten/pfsense,brunostein/pfsense,ptorsten/pfsense,BlackstarGroup/pfsense,brunostein/pfsense,phil-davis/pfsense,BlackstarGroup/pfsense,NOYB/pfsense,NewEraCracker/pfsense,jxmx/pfsense,dennypage/pfsense,dennypage/pfsense,phil-davis/pfsense