Support start and length in fcntl_lock

Sqlite and by extension cargo needs this when using eyra.

```
#10 0x00007ffff7c6e1ee in core::panicking::assert_failed (kind=core::panicking::AssertKind::Eq, left=0x7fffffff3128, right=0x7ffff7ebf5a8, args=...)
at /home/bjorn/.rustup/toolchains/nightly-2025-04-28-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/panicking.rs:380
#11 0x00007ffff7c87321 in c_scape::fs::fcntl::_fcntl (fd=6, cmd=6, args=...) at c-ward/c-scape/src/fs/fcntl.rs:89
#12 0x00007ffff7c4efa6 in c_scape::fs::fcntl::fcntl (fd=6, cmd=6, args=...) at c-ward/c-scape/src/fs/fcntl.rs:14
#13 0x0000555556a1fc45 in unixFileLock ()
#14 0x0000555556a227a0 in unixLock ()
#15 0x0000555556abca77 in sqlite3PagerSharedLock ()
#16 0x0000555556ac5afd in btreeBeginTrans ()
#17 0x0000555556b1546c in sqlite3VdbeExec ()
#18 0x0000555556b1f040 in sqlite3_step ()
#19 0x0000555556a12a4e in ::execute_batch ()
#20 0x0000555555c7920d in cargo::util::sqlite::migrate ()
#21 0x0000555555c5e3fe in ::new ()
```