plat/msdos386: refactor, firm up, and optimize

[tkchia]

• refactor code for transfer buffer reads/writes, real mode int 0x21 calls, and assembler segment declarations
• define transfer buffer size in one place
• beef up error checking for transfer buffer operations (prevent buffer overflows)
• also optimize such operations to transfer dword by dword where feasible
• rename _psp array to .psp, to avoid namespace pollution
• fix: routine for real mode int 0x21 calls was clobbering high halves of esi and edi registers

[davidgiven]

Excellent, thanks!