Try to detect system JDK/JRE home

[lbragaglia]

... instead of ~/.jenv/versions/system, at least on Debian/Ubuntu systems (see #103 and #123 ).

Let jenv to play nicely with Debian alternatives system. My use case (Debian 8.3):

$ sudo update-java-alternatives -l
java-1.7.0-openjdk-amd64 1071 /usr/lib/jvm/java-1.7.0-openjdk-amd64
java-1.8.0-openjdk-amd64 1069 /usr/lib/jvm/java-1.8.0-openjdk-amd64

$ jenv versions
* system (set by /home/lorenzo/.jenv/version)
  1.7
  1.7.0.91
  1.8
  1.8.0.66-internal
  openjdk64-1.7.0.91
  openjdk64-1.8.0.66-internal

$ java -version
java version "1.7.0_95"
OpenJDK Runtime Environment (IcedTea 2.6.4) (7u95-2.6.4-1~deb8u1)
OpenJDK 64-Bit Server VM (build 24.95-b01, mixed mode)

$ jenv javahome
/usr/lib/jvm/java-7-openjdk-amd64

$ sudo update-java-alternatives -s java-1.8.0-openjdk-amd64
[... plugin alternative errors, not relevant here ...]

$ java -version
openjdk version "1.8.0_72-internal"
OpenJDK Runtime Environment (build 1.8.0_72-internal-b15)
OpenJDK 64-Bit Server VM (build 25.72-b15, mixed mode)

$ jenv javahome
/usr/lib/jvm/java-8-openjdk-amd64

[gcuisinier]

Unfortunately, readlink -f does not work on OSX.

But It seems that greadlink from coreutils could be an alternative for osx.

[lbragaglia]

@gcuisinier thank you for the advice, I picked the function resolve_link to choose between greadlink and readlink, already used in other commands.