dev-perl/Convert-UUlib: Add IUSE="system-uulib" and disable unbundling

dev-perl/Convert-UUlib: Add IUSE="system-uulib" and disable unbundling

Gentoo's uulib implementation and the bundled version differ in some way,
making Gentoo's version less stable, causing mislinked .so files as seen
in bug #559930

This pragmatic solution just defers this problem to the user so those who
care about system purity can do that at their own cost, while making the
default workflow both Work(tm) and Work as Upstream Intended.

R1-bump required as it is expected people who have existing installations
and did not run tests will have a broken installation, which can be
confirmed by running:

  /usr/bin/perl -MConvert::UUlib=:all \
    -e'SetFNameFilter(sub {});FNameFilter(1);SetFNameFilter()'

Bug: https://bugs.gentoo.org/559930

Package-Manager: portage-2.3.0
RepoMan-Options: --ignore-arches --include-arches="alpha amd64 amd64-fbsd arm arm64 hppa ia64 m68k mips nios2 ppc ppc64 riscv s390 sh sparc sparc-fbsd x86 x86-fbsd"