Update MCSystemPick to take a StringRef

Updated for both Android and iOS (not used on desktop; Android is a nop)
Merge branch 'refactor-syntax_unicode_button' into refactor-syntax_unicode_setbuffer2
Remove MCParameter::setbuffer method

All calls to this method have been replaced with more modern equivalents.
Updates on srv multipart
Add MCNAME: an equivalent of MCSTR for NameRefs

Does exactly what it says on the tin.
Stack loading code now uses StringRefs/NameRefs
Replaced MCmessagenamestring with MCN_messagename
Some more updates on methods at 'Extras' tab
Updates after Seb's feedback
Merge remote-tracking branch 'upstream/refactor-syntax_unicode' into refactor-syntax_unicode_SrvMultipart
Add ConvertToBinary to MCExecContext

For ValueRefs that aren't already a DataRef, this converts to a string and then creates a DataRef out of the native bytes of the string.
Change MCU_puturl to use an ExecContext instead of an ExecPoint

An overload still taking the traditional EP has been provided for existing callers.
Change MCNetworkExecPutIntoUrl to no longer use GetEP

Does its thing using an ExecContext instead of an EP now.
Updated signature of cgi_get_upload_temp_dir()
Updates after Seb's feedback
Updated some methods that used GetEP
Updates after Seb's feedback #2
Merge remote-tracking branch 'upstream/refactor-syntax_unicode' into refactor-syntax_unicode_SrvMultipart
Merge remote-tracking branch 'origin/refactor-syntax_unicode_SrvMultipart' into refactor-syntax_unicode_SrvMultipart
Updates after Seb's feedback #3