Matches the host_linux change.

host_darwin does the same filtering. Not doing this gives us some rather strange
entries that likely aren't what we want.

Before:
    {"user":"reboot","terminal":"~","host":"3.10.0-327.4.5.el7.x86_64","started":1454378260}
    {"user":"LOGIN","terminal":"ttyS0","host":"","started":1454378270}
    {"user":"LOGIN","terminal":"tty1","host":"","started":1454378270}
    {"user":"runlevel","terminal":"~","host":"3.10.0-327.4.5.el7.x86_64","started":1454378276}
    {"user":"root","terminal":"pts/0","host":"vpn","started":1454404513}

After:
    {"user":"root","terminal":"pts/0","host":"vpn","started":1454404513}

Also adding a HostInfo.BootTime field for holding the boot time

Package common wasn't used for public functions. Place it in an
internal directory to prevent other packages from using.

Remove the distributed references to "HOST_PROC" and "HOST_SYS"
consts and combine into a common function. This also helps so that
if a env var is defined with a trailing slash all will continue to
work as expected.

Fixes #100

Added the ability to fetch an alternative location for /proc via an
environment variable. If the env var is not set it will return /proc as
the default value.

Note: This may not work on some old Windows XP.