Refactor local `execvpe(3)` implementation

The previous code was prone to conflicts with when the platform happens
to expose a `execvpe(3)` implementation in its libc.

This commit renames the internal implementation to `__hsunix_execvpe` as
well as adding an autoconf-detection for the presence of `execvpe(3)`,
in which case `__hsunix_execvpe()` forwards the call to `execvpe(3)`.

Moreover, the code has been cleaned up to remove likely bitrotted CPP
conditionals.

This should fix #22

(This also partially addresses #11 on platforms which have a
libc-provided `execvpe(3)`)

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