libbuxton: Set strings to const for the API

Because clients are often using "const char *" instead
of just "char *" in their internal API, it make sense
to offer this new API. Their are two main reasons:
- passing a "char*" for a "const char*" is safe
- the developer doesn't have to ask himself if removing
the const with a cast is okay or not

This change will bump our library version.
shared: Take commandline configuration option as const
libbuxton: Take set_value argument as const
libbuxton: Prefixing types with BUXTON_TYPE_

The items STRING INT32 UINT32 INT64 UINT64
REAL DOUBLE BOOLEAN are potentially conflicting.

Prefixing it is merely better.
test: Don't fail on EAGAIN write errors
shared: Improve array index bounds checking
shared: Use same index/length type for BuxtonArray
shared: Use braces in buxtonarray

Signed-off-by: Ikey Doherty <michael.i.doherty@intel.com>