**What this PR does / why we need it**:

The User-Agent reported by clients (e.g. kubectl) in request
headers should include the name of the client executable
but not the full path to that executable.

This PR changes how this name is determined by using the
operating-system specific package "path/filepath" (meant for
working with file system paths) instead of the "path" package
(meant for URL paths).

This fixes a problem on the Windows OS in the case where, if the
user has not set their PATH to point to the location of their
client executable, the User-Agent includes the full path - which
is unnecessary.

Fixes: kubernetes#44419

Automatic merge from submit-queue (batch tested with PRs 44362, 44421, 44468, 43878, 44480)

Drop leading path of KUBECTL.EXE if it shows up in User-Agent.

Partial fix for #44419 

Release note: kube-apiserver now drops unneeded path information if an older version of Windows kubectl sends it.

**What this PR does / why we need it**:

The User-Agent reported by clients (e.g. kubectl) in request
headers should include the name of the client executable
but not the full path to that executable.

This PR changes how this name is determined by using the
operating-system specific package "path/filepath" (meant for
working with file system paths) instead of the "path" package
(meant for URL paths).

This fixes a problem on the Windows OS in the case where, if the
user has not set their PATH to point to the location of their
client executable, the User-Agent unnecessarily includes the
full path.

Fixes: kubernetes#44419

Automatic merge from submit-queue

Use OS-specific libs when computing client User-Agent in kubectl, etc.

**What this PR does / why we need it**:

The User-Agent reported by clients (e.g. kubectl) in request
headers should include the name of the client executable
but not the full path to that executable.

This PR changes how this name is determined by using the
operating-system specific package "path/filepath" (meant for
working with file system paths) instead of the "path" package
(meant for URL paths).

This fixes a problem on the Windows OS in the case where, if the
user has not set their PATH to point to the location of their
client executable, the User-Agent includes the full path - which
is unnecessary.

Fixes: #44419

```release-note
Use OS-specific libs when computing client User-Agent in kubectl, etc.
```

Automatic merge from submit-queue (batch tested with PRs 44519, 43194, 44513)

Use regexp instead of substring to do search and replace.

enisoc pointed out how ToLower can change (lengthen even!) the length of
a string given arbitrary input.

Follow-up to #44421 for #44419

Automatic merge from submit-queue

Use OS-specific libs when computing client User-Agent in kubectl, etc.

This PR fixes issue 44419 in the release branch.
This PR is a patchable version of mainline PR #44423 for 1.6 release branch

The original full PR
  #44423
came after a large PR
  #40777
that split /vendor/BUILD into hundreds of BUILD files.

Thus PR 44423's version of rest/BUILD does not exist
in the 1.6 release branch, and had to be tweaked here.

```release-note
Fix for [Windows kubectl sending full path to binary in User Agent](#44419).
```

**What this PR does / why we need it**:

The User-Agent reported by clients (e.g. kubectl) in request
headers should include the name of the client executable
but not the full path to that executable.

This PR changes how this name is determined by using the
operating-system specific package "path/filepath" (meant for
working with file system paths) instead of the "path" package
(meant for URL paths).

This fixes a problem on the Windows OS in the case where, if the
user has not set their PATH to point to the location of their
client executable, the User-Agent unnecessarily includes the
full path.

Fixes: kubernetes#44419