Recognise ECOFF armap in bfd_slurp_armap

Recognise ECOFF archives and reject them so that the ecoff archive
support has a chance to handle the archive.  Also use memcmp rather
than startswith (strncmp) as we know the string length.

	* archive.c (bfd_slurp_armap): Recognize ECOFF armap.  Use memcmp
	to match names, and tidy buffer sizes.