Allow changing of MAX_RECENT in config file

Simple request made by Solarmystic on the forums. Allow you to change
the number of recent isos saved by ppsspp. Default is set to 12.
Fix names to be consistent. Moved MAX_RECENT to header file.
Merge pull request #1810 from makotech222/max-recent

Allow changing of MAX_RECENT in config file
Accept any negative channel as autoselect.

And fix error codes in sceAudioChReserve().
Auto select the highest available channel.

This is what tests show, and what the PSP does.  It makes sense, since
the game might hardcode 0/1 but some library might use autoselect.  Or
something.
Return an error for invalid audio sample sizes.
Use a separate channel for SRC/Output2.

Tests on the PSP show that all 8 channels can be reserved/used at the
same time as *either* Output2 or SRC (but SRC and Output2 cannot be used
at the same time.)
Error when trying to reserve Output2 twice.
Validate the sample count in SRC and Output2.
Validate the SRC format, and enforce STEREO.

Were these actually outputting audio before?