Increased maximum number of possible user and channel modes

Increased maximum number of possible user and channel modes

Currntly ngIRCd supports 13 user and 15 channel modes, because there
have been quite a few additions since our last release. But our data
structures can only hold 15 user and -- even worse! -- only 9 channel
modes! So enlarge the buffers to 20 bytes (actually 21 including NULL)
to allow storing of all mode characters and to have some space left
for more modes to come ...

(cherry picked from commit 8996d777621d88d4bcc439ab4792b2814920687f)