xgettext: Use sf-istream from Gnulib.
* autogen.sh (GNULIB_MODULES_TOOLS_FOR_SRC): Add sf-istream.
* gettext-tools/src/x-perl.c: Include sf-istream.h.
(struct perl_extractor): Replace fp, input, input_end fields with a single
sf_istream_t.
(phase1_getc, get_here_document, skip_pod, interpolate_keywords, extract_perl):
Update.
* gettext-tools/src/x-php.c: Include sf-istream.h.
(struct php_extractor): Replace fp, input, input_end fields with a single
sf_istream_t.
(phase1_getc): Simplify.
(process_heredoc, phase4_get, extract_php): Update.
