[core] add option to write a small enough ISO to an ESP

* This is mostly aimed at Debian 11 netinst on the Raspberry Pi 4
* Only available for regular UEFI ISOs if GPT and FAT are selected (no MBR ESPs).
* Also fix a MinGW warning in GetUnusedDriveLetter()

res/loc/ChangeLog.txt

@@ -9,6 +9,8 @@ o v3.*
   - *UPDATED*  MSG_068 "Error while partitioning drive." -> "Could not partition drive."
   You can test this new message with <Alt>-<G>
   - *NEW*      MSG_308 "VHD detection"
+  // TODO: Add a test ISO for this.
+  - *NEW*      MSG_310 "The ISO you have selected uses UEFI and is small enough to be written as (...)"
 
 o v3.5 (2019.03.12)
   The following 3 messages can be tested by creating a UEFI:NTFS drive in Rufus ('Show advanced drive properties' must be enabled

res/loc/rufus.loc

@@ -566,6 +566,10 @@ t MSG_305 "Use this option to indicate whether you want to use this device to in
 t MSG_306 "Fast-zeroing drive: %s"
 t MSG_307 "This may take a while"
 t MSG_308 "VHD detection"
+t MSG_309 "ISO → ESP"
+t MSG_310 "The ISO you have selected uses UEFI and is small enough to be written as an EFI System Partition (ESP). "
+	"Writing to an ESP, instead of writing to a generic data partition occupying the whole disk, can be preferable "
+	"for some types of installations.\n\nPlease select the mode that you want to use to write this image:"
 
 #########################################################################
 l "ar-SA" "Arabic (العربية)" 0x0401, 0x0801, 0x0c01, 0x1001, 0x1401, 0x1801, 0x1c01, 0x2001, 0x2401, 0x2801, 0x2c01, 0x3001, 0x3401, 0x3801, 0x3c01, 0x4001