top: fix warnings from clang/gcc

Add includes for <curses.h> and <termcap.h> where necessary, and
rename a few internal functions to have a "top_" prefix to avoid
clashes with standard names from curses.h/termcap.h headers.

Top now compiles without warnings on both gcc and clang.

Reviewed by: emaste, imp, jhb
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D15115