fix daemonizing: don't lose an early notify signal from the background process

[ThomasWaldmann]

The foreground process installs SIGTERM/SIGHUP/SIGINT handlers (via archiver.run) before it reaches the point where it actually waits for the background (grandchild) process to notify it via os.kill(). If the background process starts up and signals fast enough, the signal is delivered to the foreground while it is still between os.fork() and the waiting code, so the globally installed handler raises at an unexpected, uncaught place. The signal then escapes daemonizing(), bubbles up through repository teardown (NotLocked) and makes borg mount exit with rc 74.

This was observed flaky in CI with coverage's sys.monitoring backend on Python 3.14 (its first-branch lazy source parse widens the window) and the pyfuse3 backend (faster grandchild startup).

The fix blocks the notify signals before the fork in _daemonize() and waits for them atomically in the foreground. An early signal then stays pending and is reliably picked up by the wait. The background process restores the original signal mask so it keeps normal signal handling.

The wait uses signal.sigwait() plus a SIGALRM timer (signal.setitimer) rather than signal.sigtimedwait(): the latter does not exist on macOS, where it would raise AttributeError in the foreground and let it die before the background migrated the lock (breaking test_migrate_lock_alive). signal.SIGALRM in turn does not exist on Windows, where referencing it at module import time would break the import chain and the Windows PyInstaller build, so it is guarded with hasattr. Daemonizing is not supported on Windows anyway (no os.fork).

Split out of #9602, where this surfaced - the bug is independent of that PR and fixes a race that is present in master today.

[codecov]

Codecov Report

❌ Patch coverage is 41.66667% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.04%. Comparing base (91b030e) to head (db2ff30).
⚠️ Report is 5 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/helpers/process.py	41.66%	10 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10123      +/-   ##
==========================================
- Coverage   87.05%   87.04%   -0.01%     
==========================================
  Files         101      101              
  Lines       17848    17853       +5     
  Branches     2705     2709       +4     
==========================================
+ Hits        15537    15541       +4     
+ Misses       1609     1608       -1     
- Partials      702      704       +2     

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