Add scm_from_latin1_keyword and scm_from_utf8_keyword

Add scm_from_latin1_keyword and scm_from_utf8_keyword

* libguile/keywords.c (scm_from_latin1_keyword, scm_from_utf8_keyword):
  New functions appropriate for use when keyword name is a constant.

  (scm_from_locale_keyword, scm_from_locale_keywordn): Change formal
  parameter from `str' to `name'.

* libguile/keywords.h: Add prototypes for new functions.  Change formal
  parameter of scm_from_locale_keyword* from `str' to `name'.

* doc/ref/api-data.texi: Document new functions.  Remind users that
  scm_from_locale_keyword should not be used when the name is a C string
  constant.  Change formal parameter from `str' to `name'.