Add and use sbbs_free_ini() to free allocated portions of startup structures

Remove values, not keys, when setting a key with a global default.

Use UInt16 functions for getting/setting TCP ports (this fixes an issue with
the FTP PasvHigh port being set to -1 (instead of 65535).

Use iniGetUInteger instead of iniGetShortInt for most other key values.

First sbbs.ini save support in SCFG, still experimental.