Modernize macOS uninstaller: fix service management and add force mode

[tallpsmith]

Summary

This PR modernizes the macOS uninstaller to fix critical issues reported in #2347 and implements all recommendations from @natoscott.

Problem

The current uninstall-pcp script has several critical issues:

1. Obsolete service management: Uses /Library/StartupItems which hasn't existed on macOS for years
2. Missing services: Only handled pmcd and pmlogger, ignoring pmie and pmproxy
3. Outdated package removal: Used old receipt locations instead of modern pkgutil
4. Incomplete cleanup: Left ~80+ directories with "Directory not empty" errors
5. Insecure temp files: Used hardcoded /tmp paths
6. No force option: No way to do complete cleanup including config/data/logs

Changes

This PR addresses all issues through 4 logical commits:

Commit 1: Fix pmproxy plist typo

• Fixed GNUmakefile where pmproxy was incorrectly installing pmlogger.plist

Commit 2: Modernize service management

• Replace obsolete /Library/StartupItems with modern launchctl commands
• Support both bootout (macOS 10.11+) and unload for compatibility
• Add support for all 4 services: pmcd, pmie, pmlogger, pmproxy
• Use pkgutil --forget for modern package receipt management
• Explicitly remove LaunchDaemons plist files
• Preserve config/log files with user notification

Commit 3: Add force mode and mktemp

• Use mktemp(1) for secure temp directory creation
• Add -f/--force flag to skip confirmation and perform complete removal
• Add --help flag for usage information
• Implement aggressive cleanup in force mode that removes:
  • /etc/pcp/
  • /var/lib/pcp/
  • /var/log/pcp/

Commit 4: Documentation

• Add HOMEBREW_TAP_UPDATES.md with recommendations for aligning Homebrew tap
• Add TESTING_GUIDE.md with comprehensive testing procedures
• Document force mode usage and testing strategies

Testing

Standard uninstall preserves config/logs:

sudo /usr/local/libexec/pcp/bin/uninstall-pcp

Force mode removes everything:

sudo /usr/local/libexec/pcp/bin/uninstall-pcp --force

See build/mac/TESTING_GUIDE.md for comprehensive testing procedures.

Impact

• Fixes: All service management issues in #2347
• Solves: "Directory not empty" errors with force mode
• Improves: Security with mktemp
• Adds: User choice for config/log preservation
• Maintains: Backward compatibility with fallback launchctl commands

Notes

This PR is marked as DRAFT for manual testing on macOS VMs before final review.

Fixes #2347

[tallpsmith]

@natoscott I think this is good to review and merge if you're ok with it. I've tested manually both force and non-force modes in a UTM VM, they seem to be good. I am going to now aim to integrate any necessary changes into the Homebrew cask repo so that it can uninstall things cleanly there.