Replace Perl call with libcrypt

[misaochan]

• Calls libcrypt.so.1 directly via ctypes instead of spawning Perl. The password is passed as raw bytes to the C function.
• Adds a module-level threading.Lock() around the crypt() call. Without the lock, concurrent AUTHENTICATE requests caused a segfault during testing.

Libcrypt maintenance

libcrypt.so.1 is provided by libxcrypt (Ubuntu core package). This is unrelated to Python's removed crypt stdlib module or the crypt_r pypi replacement - we call the C library directly via ctypes, bypassing that issue entirely. libxcrypt is actively maintained and ships on all modern Ubuntu/Debian systems.

Tests

• Ran cmdsrv manually
• Confirmed wrongpass); echo 0 # returns "User or Password Incorrect" with no server-side execution
• Confirmed legitimate password change succeeds