lib/gshadow.c: build_list(): Improve variable and parameter names

lib/gshadow.c: build_list(): Improve variable and parameter names

It was hard to understand what each variable is.  Use a consistent
scheme, where a 'p' means a pointer, 'l' means list, and 'n' means
number of elements.  Those should be obvious from the name of the
function and the context, and will make it easier to read the code.
Also, the shorter names will allow focusing on the rest of the code.

Signed-off-by: Alejandro Colomar <alx@kernel.org>