remove unused ForgetfulDict class
avoid duplicate slashes for GENTOO_MIRRORS values
fix econf --libdir prefix path string substitutions

Previously the remaining passed arguments would still be in the variable
causing incorrect paths to be used for libdir. This fixes builds for
recent versions of sys-process/procps.
make info function output to stderr

Its only consumer (emake) gets called via command substitution in the
toolchain eclass which breaks when the info output is sent to stdout as
well.