Unconditionally build and test the ice-9/popen module.

Unconditionally build and test the ice-9/popen module.

* module/Makefile.am (ICE_9_SOURCES): Add ice-9/popen.scm.
  (ICE_9_SOURCES) [BUILD_ICE_9_POPEN]: Remove conditional addition
  of ice-9/popen.scm.
  (SCRIPTS_SOURCES): Add scripts/autofrisk.scm and
  scripts/scan-api.scm unconditionally.
  (SCRIPTS_SOURCES) [BUILD_ICE_9_POPEN]: Remove conditional addition
  of scripts/autofrisk.scm and scripts/scan-api.scm.

* configure.ac: Remove the BUILD_ICE_9_POPEN condition.

* test-suite/tests/popen.test (if-supported): Don't test for 'fork
 feature being supported.