std: allocate less memory in current_exe for OpenBSD

[joboet]

This bug was introduced back in 2f42ac4 when Alex ported the current_exe implementation from C to Rust. Vec::with_capacity measures capacity in the number of elements, but sysctl measures it in bytes, so we need to do some conversions.

CC @semarie

[rustbot]

r? @Mark-Simulacrum

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[semarie]

it seems a bit over engineered to me : sizeof(char) is 1 on all platforms that OpenBSD support.
it isn't a strong objection, but I would like to understand the rational.

[joboet]

Yes, but this isn't about an array of characters but of pointers, and size_of::<*mut c_char>() definitely isn't 1.

[workingjubilee]

Studying https://man.openbsd.org/sysctl.2#KERN_PROC_ARGS

...So uh is there a reason, since we have to make two sysctl calls, that we don't just ask for KERN_PROC_NARGV the first time?

...though "KERN_PROC_NARGV and KERN_PROC_NENV return the number of elements as an int in the argv or env array." has got to be the most curious phrasing possible for that.

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