Its development history notwithstanding, the pronounced goals of
slibtools have always included support of all reasonably modern,
reasonably posix-compliant systems and cross-targets. This commit
thus makes the distinction between 'part of the midipix framework'
and 'created by the framework's lead developer' both clearer and
more visible.
On legacy systems, posix_spawnp() might return 0 even if the execvp()
invocation in the child had failed with ENOENT.
Replace posix_spawnp() with an internal, fork+execvp based internal
function.
