Handle .old file normalization

[allanrogerr]

Description

This PR adds .old file normalization to handle cases where os.Executable() returns a path ending with .old after a previous update on some systems.

• Implements normalizeExecutablePath() function to strip .old suffix and leading dots from base filenames
• Modifies getPath() to normalize executable paths before use
• Adds comprehensive unit tests covering various path scenarios

Motivation and Context

After a self-update operation, some systems may report the current executable path with a .old extension (e.g., /path/to/.minio.old). This can cause issues when the updater
tries to determine the target path for the next update.

The implementation addresses:

1. Strip .old suffix - Removes .old extension from executable paths (lines 244-246)
2. Remove leading dots - Handles hidden files like .minio.old → minio (lines 248-250)
3. Preserve normal paths - Unchanged behavior for paths without .old extension (lines 254)
4. Cross-platform support - Works with Unix and Windows-style paths (lines 244-254)

The normalization handles:

1. Normal paths remain unchanged (lines 258-261 in test)
2. Paths with .old suffix are normalized (lines 262-265)
3. Hidden files with .old are normalized (lines 266-269, 270-273)
4. Simple filenames with .old (lines 274-281)
5. Complex paths preserved (lines 282-285)
6. Windows-style paths supported (lines 286-289)

How to test this PR?

Run the test suite:

go test -v -run TestNormalizeExecutablePath

See other tests downstream in eos here: https://github.com/miniohq/eos/actions/runs/18663370987/job/53209192041
The failures are unrelated and are being discussed with @vadmeste in https://github.com/miniohq/eos/issues/1956

Types of changes

[x] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Optimization (provides speedup with no functional changes)
[ ] Cleanup/Maintenance (no functional changes)
[ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

[ ] Fixes a regression (If yes, please add commit-id or PR # here)
[x] Unit tests added/updated
[ ] Internal documentation updated
[ ] Create a documentation update request

[harshavardhana]

@ramondeklein this fixes the odd issues that we have seen during Operator drive minio upgrades.