foreign-string-alloc and :string should be able to ignore errors

[Luís Oliveira]

Currently, the following is possible:
  (let ((babel-encodings:*suppress-character-coding-errors* t))
    (foreign-string-alloc "pão" :encoding :ascii))

However, it's not very convenient, particularly when using :string types. Even more so, if the :string type is used in a callback function.

Feature requested by Michael Raskin: <http://thread.gmane.org/gmane.lisp.cffi.devel/1852>