Fix buffer overflow in feature parsing

Fix buffer overflow in feature parsing

String features (variants, egt, myname and option) relied on a buf[MSG_SIZ]
for processing their value. The Nebiyu engine had combobox options that
required more than that. All string features are now stored in allocated
memory rather than in static arrays, and StringFeature allocates a buffer
of sufficient size for them. Only limitation now is the low-level input
buffer in the InputSource threads, whih is a static buffer of 4096
(=INPUT_SOURCE_BUF_SIZE) characters.