add missing return statement for boot time value retrieved from stat file. Also move current time fetch to be closer to where the "time since boot file" is read
`VERSION_ID` is more appropriate for scripts and other usages, since `VERSION` can contain spaces and codenames
from `os-release` manpage:
```
VERSION=
A string identifying the operating system version, excluding
any OS name information, possibly including a release code
name, and suitable for presentation to the user. This field
is optional.
Examples: "VERSION=17", "VERSION="17 (Beefy Miracle)"".
VERSION_ID=
A lower-case string (mostly numeric, no spaces or other
characters outside of 0–9, a–z, ".", "_" and "-") identifying
the operating system version, excluding any OS name
information or release code name, and suitable for processing
by scripts or usage in generated filenames. This field is
optional.
Examples: "VERSION_ID=17", "VERSION_ID=11.04".
```
By assuming virtualization environment won't change during a the program's runtime, we can cache common/common_linux.Virtualization() with a simple map to reduce amount of system calls. I first mentioned this issue at https://github.com/shirou/gopsutil/pull/890#issuecomment-690211919
gopsutil is a transitive dependency of another project that I am integrating
into an internal build system. We target multiple platforms and as a part
of the build system for the large internal repo, we calculate the build
graph used to determine what targets have changed and need to be build /
tested as a single DAG for all platforms.
gopsutil currently does not form a DAG if linux and any other platform are
considered at the same time. linux is the only platform where the process
package imports the host package.
To remove this cycle, the relevant methods have been moved to internal/common
with the linux build tag and are consumed the host and process packages.
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