build-sys: default to gold linker

gold doesn't exhibit the problems with linking of compatibility
libraries.

It is also slightly faster:

make clean && make -j5   bfd       gold

real                   34.885s     33.707s
user                   34.486s     32.189s
sys                    9.929s      10.845s

real                   35.128s     33.508s
user                   34.660s     31.858s
sys                    10.798s     10.341s

real                   35.405s     33.748s
user                   34.765s     32.384s
sys                    11.635s     10.998s

real                   35.250s     33.795s
user                   34.704s     32.253s
sys                    11.220s     11.469s

touch src/libsystemd/sd-bus.c && make -j5
                        bfd        gold

real                   10.224s      9.030s
user                   11.664s      9.877s
sys                    3.431s       2.878s

real                   10.021s      9.165s
user                   11.526s      9.990s
sys                    3.061s       3.015s

real                   10.233s      8.961s
user                   11.657s      9.973s
sys                    3.467s       2.202s

real                   10.160s      9.086s
user                   11.637s      9.950s
sys                    3.188s       2.859s
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
build-sys: disable lto for compatibility libraries

https://sourceware.org/bugzilla/show_bug.cgi?id=16504
build-sys: create "compatibility libraries" section

Compat stuff is moved to src/compat-libs/.
Warnings are issued when programs are linked with the deprecated library.
build-sys: merge libsystemd-id128 into libsystemd