RFR(S): 8025942: Implement os::Bsd:available_memory

[Gerard Ziemski]

Please review this fix that implements os::Bsd:available_memory()

Description:

This is a simple change - we use a similar implementation that Apple 
itself uses (see 
http://opensource.apple.com/source/system_cmds/system_cmds-498.2/vm_stat.tproj/vm_stat.c) 
to implement BSD (Apple platform only) implementation for finding out 
available (free) memory.


Testing:

Passed JPRT, vm quicklooks


References:

http://cr.openjdk.java.net/~dsamersoff/sponsorship/gziemski/JDK-8025942/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8025942


cheers