Clean up filelock code related to errno

Clean up filelock code related to errno

Reduce dependency on Haiku internals, by not assuming that
Haiku errno values (which are negative) are neither -1 nor -2.
This removes an #ifdef HAIKU while still maintaining
portability to Haiku.
* src/filelock.c (NEGATIVE_ERRNO, ANOTHER_OWNS_IT, I_OWN_IT):
New constants, which should work regardless of whether
we are on Haiku or B_USE_POSITIVE_POSIX_ERRORS is defined.
(current_lock_owner, lock_if_free, lock_file, unlock_file)
(Ffile_locked_p): Use them, without assuming anything about errno
value sign.