fix localeconv field value for unavailable values

per ISO C, CHAR_MAX, not -1, is the value used to indicate that a char
field in struct lconv is unavailable.

patch by Julien Ramseier.

include cleanups: remove unused headers and add feature test macros

fix localeconv values and implementation

dynamic-allocation of the structure is not valid; it can crash an
application if malloc fails. since localeconv is not specified to have
failure conditions, the object needs to have static storage duration.

need to review whether all the values are right or not still..

initial check-in, version 0.5.0