[jnc_io_base] don't rely on system execvpe (missing on mac and GLIBC < 2.11)
[jnc_io_base] don't rely on system execvpe (missing on mac and GLIBC < 2.11)

instead, use simple implementation via execvp (the one we currently use
on mac)

vovkos/jancy,vovkos/jancy,vovkos/jancy,vovkos/jancy,vovkos/jancy