feature: make `pathrs_inroot_open` support `O_CREAT`

[cyphar]

C programmers are used to open(2) accepting O_CREAT so we should probably emulate the same support in libpathrs. This would just mean that pathrs_inroot_open(O_CREAT) calls pathrs_inroot_creat() transparently.

[cyphar]

This will also critically make us let Root.Create actually match os.Create (i.e., it implies O_TRUNC and is just a wrapper around OpenFile(..., os.O_CREAT|os.O_TRUNC|os.O_RDWR, 0o666)).

[cyphar]

Ah right, this is problematic because of the whole "mode argument" issue and the lack of support for va_args in Rust (not to mention that the whole API kinda sucks).