Fix rescanning when a macro expansion result forms a new function-like macro call

[pfultz2]

When the result of a macro expansion ends with the name of a function-like
macro and the tokens that follow supply its arguments, simplecpp performed
that rescan only once instead of repeatedly. Given:

#define a(b, c) c
#define d() a
#define g(e) h(e, ) h(e, )
#define h(e, b) d()(, e)()
#define i()
g(i)

g(i) expanded to i ( ) i ( ) instead of nothing: inside h's body
d()(, e)(), d() was correctly rescanned as a(, i) yielding i, but the
resulting i was never rescanned against the trailing ().

The rescan logic was also missing logic to prevent re-expanding the same macro name: so #define f() f + f()() gave f instead of f().