Avoid allocating in pre_exec closure

[purplesyringa]

Error::new allocates memory (see rust-lang/rust#148971). This is bad in multi-threaded programs, which virtual-terminal as a library can be used in. If the fork occurs while the allocator lock is held by another thread, deadlocks can occur, since there's no one left in the new process to unlock the mutex. I do not believe this is UB, and modern libc offer protections against this issue, but this isn't POSIX-compliant and should preferably be avoided.

last_os_error does not allocate, but only supports error codes. However, the old method swallowed both the text and the error code, so hopefully that isn't a problem. If necessary, a valid approach to transmit an error would be to write to stderr (e.g. by panicking), but this is ugly.

[divi255]

Good day,

thanks for reporting I will review and merge this soon. btw to avoid additional allocations in realtime and other similar programs, heap preallocation is also useful (see https://docs.rs/rtsc/latest/rtsc/thread_rt/fn.preallocate_heap.html)