Daniel Verkamp: COMBOOT Support

Implement file I/O functions (open, read, close) based on gPXE's posix_io support dcf8448c1e922892ee8dc4cf7a238eaebca1a5df

Finally debugged why my VGA test case (a 256-byte demo, tube by 3SC ) wasn't working - it was assuming that registers were 0, and I hadn't been clearing them; after taking care of that, it works fine in bochs, qemu, and real hardware: 169e9975ba0aea887bf5515f9bbe32466bf3b5e0

Fixed a bunch of small things in com32_call.c, after which basic file I/O (filetest.c32) works d645d491cf48d7ef48566057b9b8ad6c5418b5d9

Implement more INT 22h functions (AX = 0005h, AX = 0007h, AX = 0013h, AX = 0015h) 068eb73795d962cc70336a8159e6e91ecbd5cb68

Implement “Resolve hostname” function; resolv.c32 now works 01fa277122c8ee33e26a35b7ef890e57b8b485e3

Map Enter key to 0x0D instead of 0x0A; now fancyhello.c32 works d5f9318b8570cf3c489e4d3f8dfe1955cb837024

Rebased comboot work on top of current upstream gPXE tree and pushed to comboot-merge branch