Implement bashreadline with libbpf.

Implement bashreadline with libbpf.

Bashreadline will print user inputs, returning from readline, of every
instance of bash shell.  Readline is in bash itself, linked
statically, for some devices, while others may link to libreadline.so.
This implementation finds the symbol in bash if possible. Or, it tries
to find libreadline.so using ldd if the symbol is not in bash.

Signed-off-by: Kui-Feng Lee <kuifeng@fb.com>