Add sysctl to query amount of swap space free
Add sysctl to query amount of swap space free

PR:		kern/166780
Submitted by:	Radim Kolar <hsn@sendmail.cz>
Approved by:	cperciva
MFC after:	1 week

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase