dev-db/firebird: Restore SuperClassic, fix broken symlinks, libfbintl.so

dev-db/firebird: Restore SuperClassic, fix broken symlinks, libfbintl.so

Gentoo-Bug: 577956, 562716, 516760

1) Restore ability to build SuperClassic server

Commit 9b00135 ("Various syntax improvements and fixes") removed
ability to build the SuperClassic server by changing REQUIRED_USE so
that the required USE flags combination ("-client -superserver -xinetd")
is no longer permitted. It looks like this was done by mistake, since the
relevant code inside ebuild is still present and still works.

Fix this by allowing this USE flag combination again.

2) Portage warns about broken symlinks when firebird is emerged:
* QA Notice: Symbolic link /usr/lib64/firebird/lib/libib_util.so points to
/usr/lib64/firebird/lib/usr/lib64/libib_util.so which does not exist.
* QA Notice: Symbolic link /usr/lib64/firebird/intl/fbintl.conf points to
/usr/lib64/firebird/intl/etc/firebird/fbintl.conf which does not exist.
* QA Notice: Symbolic link /usr/lib64/firebird/intl/fbintl points to
/usr/lib64/firebird/intl/usr/lib64/libfbintl.so which does not exist.
* QA Notice: Symbolic link /usr/lib64/firebird/plugins/libfbtrace.so
points to /usr/lib64/firebird/plugins/usr/lib64/libfbtrace.so which does
not exist.

Adjust symlink targets in ebuild to fix these broken links.

3) Don't use libintl.so SONAME for libfbintl.so

Firebird by default uses libintl.so.1 SONAME for libfbintl.so.1.
This means that ldconfig will make libintl.so.1 symlink to libfbintl.so.1.

However, actual libintl.so is a very different library, incompatible with
libfbintl.so. This breaks packages which tries to link to it, notably
dev-perl/libintl-perl.

Fix it by making libfbintl.so SONAME match its actual name.

Package-Manager: Portage-2.3.3, Repoman-2.3.1

Closes: https://github.com/gentoo/gentoo/pull/3757