[PATCH] fixes for android in libraries/unix

[Nathan Hüsken]

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.