procfs: add KernelTainted() to parse /proc/sys/kernel/tainted

[arunsrini3082]

Add KernelTainted() method on FS that reads /proc/sys/kernel/tainted
and returns the raw taint value along with each known flag parsed into
a KernelTainted struct.

This is required by prometheus/node_exporter to implement a tainted
kernel collector, which was originally proposed in
prometheus/node_exporter#1567 but not merged
because parsing was expected to live in procfs first.

The struct exposes:

• Value: raw uint64 from /proc/sys/kernel/tainted
• Bits: slice of KernelTaintBit with Index (bit position), Flag (letter
  code), Description, and Set (whether currently active)

Reference: https://www.kernel.org/doc/html/latest/admin-guide/tainted-kernels.html

[SuperQ]

Something is wrong with the testdata change.

[arunsrini3082]

@SuperQ Thanks for the suggestions! I've applied both and reconciled them — since SysReadUintFromFile already returns uint64, I updated parseTainted to accept uint64 directly (no more io.Reader or string parsing). The "io", "fmt", "strconv", and "strings" imports are now removed, and the test updated accordingly. Pushed in the latest commit.