Correct problems with IMAP parsing when using Turkish: Bug #714892

When connecting to any IMAP server while the local user's locale is
configured to be Turkish, Geary will mis-parse many of the IMAP
server's responses, leading to essentially a failed connection due to
state issues and more.

The problem is that some of the parsing code was using g_utf8_strdown
to convert received text to lowercase to perform case-insensitive
string comparisons.  Turkish has multiple letter I's (dotted and
dotless), and when the UTF-8 code transformed it to lowercase, a
different UTF-8 code point was selected than the English/ASCII 'i'.

The solution is to explicitly use ASCII variants of string
transformation, comparison, and hashing to ensure 7-bit operations are
used throughout the IMAP and RFC822 stack.  Further commits will
follow that enforce this a bit more, but this commit is sufficient to
correct the problem for our Turkish users.