Add NumFDs implementation for Darwin

[kerlenton]

Implements NumFDs() for macOS using the proc_pidinfo syscall with PROC_PIDLISTFDS.

Changes

• Added NumFDsWithContext() implementation for Darwin in process_darwin.go
• Uses two-stage syscall pattern (standard for macOS APIs):
  1. First call gets required buffer size
  2. Second call retrieves actual FD list
• Added PROC_PIDLISTFDS constant to common_darwin.go
• Moved NumFDsWithContext stub from process_bsd.go to OS-specific files
• Added tests following existing test patterns

Testing

• Tests pass on macOS
• Correctly counts file descriptors
• Handles non-existent processes

[shirou]

Looks great overall! Thank you for your contribution!