Added file sealing for making `-exec/{}` better (esp. for memfile mode.)
Fixed `errors` being shit.
Removed `memfile-preallocate` from `mode-memfile` feature: It is unneeded.
Fortune for collect's current commit: Middle blessing − 中吉
TODO: Should `-exec{} {}` be valid? That is, should we allow the use of `fexecve()` instead of `execve()` in the special case where the command itself of a positional exec is the positional replacement string? This may be useful, but also dangerous and perhaps not possible with a memfd or with a pipe. We"d also need to `ioctl()` set to permissions to RX instead of RO, and any subsequent instances of the replacement string *should probably* be `dup()`"d file-descriptors with -X permission set. Note for later. For now, a warning is ommitted if this is present, explaining to the user that executing the input is not (yet??) supported.
Fortune for collect's current commit: Future small blessing − 末小吉
Motivation for this addition is that often shells will run processes in a pipeline in parallel, regardless of if they have any input yet. This leads to a lot of messy `ifne` useage that is unneeded with this option. It also allows for greater efficiency in memfd mode as the process passed the /proc fd can be spawned before the copy to stdout (TODO: behaviour of this still undefined.) and the `memfd_create()` fd can be used instead of tweaking stdout to be used.
This essentially functions the same way as shell process substitution but with an actually complete file; it waits for all input to be collected *before* spawning the process. This can be useful for interactive programs like gpg, which often are not fond of waiting for input and want it all immediately. Programs that map files into memory can also not like being passed pipe fds, in memfd mode, this shouldn"t be a problem, the fd should be shareable immediately with the correct `ioctl()` call (or perhaps even without.)
Fortune for collect's current commit: Small blessing − 小吉
Needs testing, and the XXX comment about `memfd_create()` name lifetimes also needs testing. (If they do need to be static; `stackalloc` as a dependancy can be removed entirely, then: `DEFAULT_NAME` will be changed to a CString, function parameter `name` and `error::MemfileCreationStep::Create(name)` will be changed to `Option<&"static CStr>`.
Fortune for collect's current commit: Future blessing − 末吉
Avril