Make disk free space reporting consistent with Linux 'df' command.

Make disk free space reporting consistent with Linux 'df' command.

On Linux, statfs syscall reports free space in two fields.
From http://man7.org/linux/man-pages/man2/statfs.2.html:
1. f_bfree => Free blocks in filesystem
2. f_bavail => Free blocks available to unprivileged user

For file systems that maintain reserved space for system
activities f_bfree > f_bavail because f_bfree includes the
reserved space in the file system. The reserved space is
typically 5% for 'ext4' file systems. Newer distros like
RHEL 7.x use 'xfs' by default and report same value for
both the fields.

The Linux 'df' command uses f_bavail in its reporting.
Tools reports, conditionally, f_bfree for root and f_bavail
for non-root.  However, since vmtoolsd runs as root, Tools
always reports f_bfree, which is more free space than 'df'
would report (depending on amount of reserved space).

In order to be consistent with Linux 'df' command,
report f_bavail as the disk free space by default. This does
change the behavior a little bit in that Tools will report less
disk free space than before, the difference being the same as
the reserved space on the file system, typically 5%. This
should be OK in general because it makes the space reporting
a bit conservative. If this change in behavior is not desired
for some use cases, the old behavior can be restored by setting
the following newly added configuration in this change:

[guestinfo]
diskinfo-include-reserved=true

The existing callers that are outside the guestInfo plugin
will continue to include reserved space in their space
accounting as before.

Also fixed a few minor stuff/touchups in vmtoolsConfig.c.