Proposed rule: result of strcmp should not be treated as boolean

Taken from: https://www.teamten.com/lawrence/style/notes.html

Bad:   if (!strcmp (a, b)) ...
Good:  if (strcmp (a, b) == 0) ...

Don't use the ! operator on values that are not booleans. The "strcmp" function returns an integer which is less than 0 if a < b, 0 if a == b, and greater than 0 if a > b. (Note: it does not return -1, 0, 1.) The ! operator here is not faster and is misleading because the line reads, "if not string compare" whereas it means "if the strings compare." The same is true about pointers – don't say "if (!ptr)" when "if (ptr == NULL)" is just as fast and much more readable, and same with "if (ptr)" when "if (ptr != NULL)" is better.