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aux/vga: dont use /proc/$pid/mem to access vga bios

using /proc/$pid/mem to access vga bios is not portable and crashes
sgi machines when aux/vga is run. instead, try /dev/realmodemem
first (provided by realemu), then #v/vgabios.

--- a/sys/src/cmd/aux/vga/io.c

+++ b/sys/src/cmd/aux/vga/io.c

@@ -12,7 +12,6 @@

 static int iolfd = -1;

 static int biosfd = -1;

 static int msrfd = -1;

-static ulong biosoffset = 0;

 enum {

 	Nctlchar	= 256,

@@ -243,20 +242,13 @@

 static long

 doreadbios(char* buf, long len, long offset)

 {

-	char file[64];

-

-	if(biosfd == -1){

-		biosfd = open("#v/vgabios", OREAD);

-		biosoffset = 0;

-	}

-	if(biosfd == -1){

-		snprint(file, sizeof file, "#p/%d/mem", getpid());

-		biosfd = devopen(file, OREAD);

-		biosoffset = 0x80000000;

-	}

-	if(biosfd == -1)

+	if(biosfd < 0)

+		biosfd = open("/dev/realmodemem", OREAD);

+	if(biosfd < 0)

+		biosfd = devopen("#v/vgabios", OREAD);

+	if(biosfd < 0)

 		return -1;

-	seek(biosfd, biosoffset+offset, 0);

+	seek(biosfd, offset, 0);

 	return read(biosfd, buf, len);

 }