font.eclass: Don't assign FONT_S in global scope, allow an array.

font.eclass: Don't assign FONT_S in global scope, allow an array.

Accessing ${S} in global scope is not allowed by PMS, therefore remove
the global variable assignment of FONT_S which uses it. Add a fallback
to ${S} in font_src_install() instead.

Allow FONT_S to be an array, if there are multiple directories.
Support for whitespace-separated lists will be kept for some time,
and a QA warning will be shown.

Die if pushd or popd fails.

Closes: https://bugs.gentoo.org/613108
Closes: https://bugs.gentoo.org/709578
Signed-off-by: Ulrich Müller <ulm@gentoo.org>