[PATCH] fixes for android in libraries/unix

Hey, These patches are for libraries/unix. They may need some discussing, because I am not sure if this is the right thing. 1. On android the passwd struct does not define pw_gecos. The first patch therefor sets it to the empty string. 2. The android ndk does not define _POSIX_VDISABLE, but it defines _PC_VDISABLE. The second patch defines _POSIX_VDISABLE to _PC_VIDISABLE, which is (as I undestand it) what the POSIX programmer guide suggests. 3. The android ndk does not define telldir and seekdir. The third patch removes them when compiling for android.