libeinfo: add separate variable to suppress eerror messages

Add an EERROR_QUIET environment variable which works like EINFO_QUIET
but for the eerror functions. This will allow library consumers to
choose whether to suppress eerror messages separately from einfo and
ewarn messages.

X-Gentoo-Bug: 482396
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=482396