* tta/C/convert/convert_to_info.c (info_format_anchor),
tta/C/convert/convert_to_plaintext.c (PLAINTEXT_FORMAT_FUNCTIONS)
(plaintext_format_anchor, plaintext_functions)
(convert_to_plaintext_internal), tta/perl/Texinfo/Convert/InfoNonXS.pm
(format_anchor), tta/perl/Texinfo/Convert/PlaintextNonXS.pm
(format_anchor, _convert): rename _anchor as format_anchor, move
definition to Info converter, the plaintext implementation being a
no-op.  Add dispatch information in C.

* tta/perl/Texinfo/Convert/PlaintextNonXS.pm (process_footnotes),
tta/C/convert/convert_to_plaintext.c (plaintext_process_footnotes):
create a footnote anchor only if target_locations are output.