Android hints: Detect stub functions and mark them as undefined.

Android hints: Detect stub functions and mark them as undefined.

Bionic implements several functions as stumps that simply warn
and return null, which leads to situations like this:

FIX ME! implement getprotobyname() bionic/libc/bionic/stubs.c:378

This commit introduces some probes for functions (getnetbyname,
getnetbyaddr, getmntent, getprotobyname, getprotobynumber, endpwent,
ttyname) that we use and are actually stubs in at least some
versions of Android.
If we find a stub, we pretend that the function is undefined, so
the above example becomes this:

Unsupported socket function "getprotobyname" called at -e line 1.

Note that this commit does not fix the modules that assume that
these functions are always available, so 'make test' still fails.