rewrite open-process in C, for robustness

rewrite open-process in C, for robustness

* libguile/posix.c (scm_open_process): Rewrite in C, so as to avoid
  allocations and other calls that are not async-signal-safe.
  (scm_init_popen, scm_init_posix): Register popen extension.

* module/ice-9/popen.scm: Load the popen extension, to get open-process.