A execve() wrapper to fix problem with shebangs when running in Termux.

A lot of Linux software is written with the assumption that /bin/sh, /usr/bin/env and similar file exists. This is not the case on Android where neither /bin/ nor /usr/ exists.

When building packages for Termux those hard-coded assumptions are patched away - but this does not help with installing scripts and programs from other sources than Termux packages.

Create an execve() wrapper that rewrites calls to execute files under /bin/ and /usr/bin into the matching Termux executables under $PREFIX/bin/ and inject that into processes using LD_PRELOAD.

1. Install with pkg install termux-exec.
2. Exit your current session and start a new one.
3. From now on shebangs such as /bin/sh and /usr/bin/env python should work.

The $PREFIX/bin/login program which is used to create new Termux sessions checks for $PREFIX/lib/libtermux-exec.so and if so sets up LD_PRELOAD before launching the login shell.