basic,shared: move dlopen helpers to shared/

basic,shared: move dlopen helpers to shared/

This was added in 88d775b734644f26fb490836769c2bc275498fde,
with the apparent intent of using in shared/ and the rest of our code.
It doesn't matter much for our code, since libdl is part of glibc anyway,
but moving it removes one linkage from libsystemd. (libshared was already
linking to libdl explicitly).