Write a strcspn() function that works as follows. strcspn() has two
string arguments, s and f. It starts at the beginning of s, searches
through s for a character contained in f, and returns the position of
that character (i.e., the number of characters before it in s).

For example, strcspn("HELLO","ABCDE") returns 1; strcspn("HELLO","L")
returns 2; strcspn("HELLO","GF") returns 5.