N wildcards in adapter should never incur a mismatch

[marcelm]

Currently, an adapter such as -g ^NNACGT will match a read that starts with NTACGT with one mismatch since the first N in the read is counted as a mismatch.

The is unexpected. Instead, all N wildcards in an adapter sequence should match whatever character is in the read at the corresponding position. (Unless --no-match-adapter-wildcards/-N has been used.)