Adjust IsIdentValid test to comply with upcoming ircd-hybrid release (#272).

Adjust IsIdentValid test to comply with upcoming ircd-hybrid release (#272).

hybrid.cpp: adjust IsIdentValid test to comply with upcoming ircd-hybrid where
user names may only consist of [A-Za-z0-9._-] and may not begin with '.', '-',
or '_'.

As a side effect this fixes an issue where currently it is possible to introduce
bots whose user name starts with (non-alnum) characters considered invalid for
ircd-hybrid leading to a services shutdown due to a nick introduction/kill loop.