RFR : 8195857 : Posix platforms : move os::is_headless_jre check coding to os::Posix and remove test for libmawt

[Baesken, Matthias]

Hello,  I  noticed that  os::is_headless_jre()  still checks  on the Posix platforms  for libmawt  which is not present any longer.
Additionally the  os::is_headless_jre()   function could be  put in a central place at os_posix  to avoid code duplication.

Could you please review my change .

Webrev :

http://cr.openjdk.java.net/~mbaesken/webrevs/8195857/

Bug :

https://bugs.openjdk.java.net/browse/JDK-8195857



Thanks, Matthias