How to populate a config struct is decided by one or several
config_mapping arrays where at each index there is listed:
* the key that is looked for in the config
* the type
* the offset of the field in the target struct
* one of the following union fields:
  + sm - a submap for nested structs
  + op - a struct with three function pointers for reading, writing, and releasing memory of a custom datatype

Five types exist, but they are extensible:

's'      - string, the rest of the config line is read
'i', 'u' - signed and unsigned integers, 32 bits are default, but you can change this by changing CFG_INTEGER_BITS
           to some other 8/16/32/64 value, or set it to e.g. 0 and define a number of custom macros, see config_core.h
'b'      - boolean (bool), the loaded value depends on if the first value character is 'y' or 'n' (case insensitive)
           when output is generated either "yes" or "no" (without quotes) will be printed.
'*'      - custom datatype, you need to provide read/write/memory release functions yourself
'+'      - nested struct, you need to provide a config submap

The config file format is simple (ws* being zero or more whitespace characters):

ws* KEY ws* = ws* DATA

for s/i/u/* types. s stops at the the end of the line, and the value may thus contain spaces.
There is enforced ending condition for the custom datatypes.

For nested structs the format is

ws* KEY ws* = ws* { eol
	SUBSTRUCT
	...
ws* } eol

where eol is end of line, and SUBSTRUCT is the appropriate key-value pairs for the substruct, following
the same format as any other level.

A line where the first non-whitespace character is a '#' is considered a comment and ignored.

The example code shows off both a custom datatype as well as the nesting capability.