Have `scm_take_locale_symbol ()' return an interned symbol (fixes bug #25865).

Have `scm_take_locale_symbol ()' return an interned symbol (fixes bug #25865).

* libguile/symbols.c (intern_symbol): New function, with code formerly
  duplicated in `scm_i_c_mem2symbol ()' and `scm_i_mem2symbol ()'.
  (scm_i_c_mem2symbol, scm_i_mem2symbol): Use it.
  (scm_take_locale_symboln): Use `intern_symbol ()'.  This fixes
  bug #25865.

* test-suite/standalone/Makefile.am
  (test_scm_take_locale_symbol_SOURCES,
  test_scm_take_locale_symbol_CFLAGS,
  test_scm_take_locale_symbol_LDADD): New variables.
  (check_PROGRAMS, TESTS): Add `test-scm-take-locale-symbol'.