implement POSIX setenv() to change app's env vars, which GnuPG requires

This allows GPG to set the env vars at app launch, and everything should
magically get the env. There are two things that require setting env vars:

* Android's linker can only find shared libs by looking in LD_LIBRARY_PATH,
  so in order for the GnuPG command line tools to find the required shared
  libraries, LD_LIBRARY_PATH needs to include the apps lib paths also.

* GnuPG needs env vars like GNUPGHOME to find the folder where keyrings,
  UNIX sockets, and config files are kept.

http://stackoverflow.com/questions/318239/how-do-i-set-environment-variables-from-java

#1548