Smaller C: Improve pointer initialization

The following global/static pointer initialization should now work:

static char* p1 = "abc" + 1;
static char* p2 = "abc" - 1;
static char* p3 = 1 + "abc";
static int a333[3][3][3];
static int* pa333_111 = &a333[1][1][1];

Other seemingly (and in fact actually) functionality preserving changes
are only to reduce code size when self compiling for DOS using -seg16.
Improvements and fixes require space, sigh.
Merge pull request #19 from alexfru/master

Smaller C: Improve pointer initialization