lib-settings: settings_read*() now returns an error string instead of logging itself

Removed CONTEXT_TYPE_SAFETY macro and reimplemented its functionality better. gcc/clang now gives a compiler error in many places if callback isn't exactly what was expected. It's also now much easier to add more of these checks.

Marked functions parameters that are allowed to be NULL. Some APIs were also changed. The non-obvious APIs where NULL parameter was changed to "" are master_service_init() and auth_master_user_list_init(). These checks can currently be enabled only on a patched clang: http://llvm.org/bugs/show_bug.cgi?id=6786

Changed all settings related structs to const and changed APIs to make it work. --HG-- branch : HEAD

Changed .h ifdef/defines to use <NAME>_H format. --HG-- branch : HEAD

Added type checks to setting defines. --HG-- branch : HEAD

Type safe callbacks weren't as easy as I thought. Only callback(void *context) can be handled generically. Others can be handled specially, but only if all the parameters are pointers, otherwise eg. int parameter can be replaced with long without compiler giving any warnings. --HG-- branch : HEAD

Added context parameter type safety checks for most callback APIs. --HG-- branch : HEAD

Added "bool" type and changed all ints that were used as booleans to bool. --HG-- branch : HEAD

New configuration file code. Some syntax changes, but tries to be somewhat backwards compatible. SIGHUP now reverts back to old configuration if it detected errors in new one. --HG-- branch : HEAD

updates --HG-- branch : HEAD

Moved settings parsing to lib-settings. --HG-- branch : HEAD