xgettext: Scheme: Understand both '-L Scheme' and '-L Guile'.

xgettext: Scheme: Understand both '-L Scheme' and '-L Guile'.

* gettext-tools/src/x-scheme.h (EXTENSIONS_SCHEME): Associate the language
Guile, not Scheme, with the extension .scm.
(SCANNERS_SCHEME): Add info about the 'Guile' language.
(extract_guile): New declaration.
* gettext-tools/src/x-scheme.c (follow_guile): New variable.
(extract_whole_file): Renamed from extract_scheme.
(extract_scheme, extract_guile): New functions.
* gettext-tools/src/xgettext.c (usage): List Guile as a possible argument for
the '-L' option.
* gettext-tools/doc/xgettext.texi: Document 'Guile' as possible argument for
the '-L' option.
* gettext-tools/doc/lang-scheme.texi (Scheme): Explain the difference between
'-L Scheme' and '-L Guile'.
* NEWS: Mention the change.