build-sys: merge libsystemd-login into libsystemd

build-sys: merge libsystemd-login into libsystemd

A compatibility libsystemd-login library is created which uses
.symver and ifunc magic proposed by Lennart to make programs linked
to the old library name continue to work seamlessly.

Unfortunately the bfd linker crashes:
  https://sourceware.org/bugzilla/show_bug.cgi?id=16467
This will be fixed in binutils 2.25.

As a work-around, gold can be used:
  LDFLAGS=-Wl,-fuse-ld=gold

Unfortunately the switch to pick the linker appeared in gcc 4.8.

This also doesn't work with LLVM:
  http://llvm.org/bugs/show_bug.cgi?id=11897