(STRING_SYSCALL, STRING2_SYSCALL): New helper macros.

(STRING_SYSCALL, STRING2_SYSCALL): New helper macros.
Use them instead of SCM_SYSCALL when one or two strings need to be
converted into locale strings.
(my_rename): New, gathers platform dependent code for renaming.
(scm_rename): Use it.
(scm_readlink, scm_copy_file): Convert SCM strings to locale
strings instead of accessing their internals.
(scm_basename, scm_dirname): Use SCM_I_STRING_CHARS and
SCM_I_STRING_LENGTH instead of SCM_STRING_CHARS and
SCM_STRING_LENGTH.