memcmp: avoid doing single char memcmp

memcmp: avoid doing single char memcmp

There is no real gain in performing memcmp() comparisons on single
characters, so change these to array subscript inspections which
saves a call and makes the code clearer.

Closes #3486
Reviewed-by: Daniel Stenberg <daniel@haxx.se>
Reviewed-by: Jay Satiro <raysatiro@yahoo.com>