Cleanup configuration parsing

[fluca1978]

This is somehow similar to #238: in the configuration.c file the function pgagroal_read_configuration is full of blocks like this one:

               if (!strcmp(key, "host"))
               {
                  if (!strcmp(section, "pgagroal"))

that checks for the host key parameter outside of the pgagroal section.
It seems to me the blocks are repeated quite a lot, and somehow unconsinstently. For example at https://github.com/agroal/pgagroal/blob/master/src/libpgagroal/configuration.c#L270 there is an extra check for section to be non empty, that is not present in other parts, that instead do an explicit if ... else on section (e.g., https://github.com/agroal/pgagroal/blob/master/src/libpgagroal/configuration.c#L247.

My proposal is to create two different functions:

• pgagroal_key_in_main_section( key ) will return true if key is defined within the [pgagroal] section;
• pgagroal_key_in_section( key, section ) will return true if the key is defined in a custom user-defined section.

Therefore, this code:

               if (!strcmp(key, "host"))
               {
                  if (!strcmp(section, "pgagroal"))
                  {
                     // A  ...
                 }
                  else if (strlen(section) > 0)
                  {
                     // B   ....
                  }
                 else
                 { 
                    unknown = true;
                }

can be rewritten as

if ( pgagroal_key_in_main_section( "host" ) )
{  
   // A ...
} 
else if ( pgagroal_key_in_section( "host", section ) )
{
   /// B ...
}
else
{
   unknown = true;
}

Clearly this does not change the behavior of the configuration, but removes repetitions and should make pgagroal_read_configuration() more simple to new configuration parameter additions.