Cleanup handling of interrupt controller (PIC).

Add new file (pic.h) with helpers for accessing the PIC.
Move irq enabling code to the files that use those interrupts.
Also, don't setup for floppy if floppy support not enabled.
diff --git a/src/system.c b/src/system.c
index 0941565..e73fe5c 100644
--- a/src/system.c
+++ b/src/system.c
@@ -9,6 +9,7 @@
 #include "biosvar.h" // BIOS_CONFIG_TABLE
 #include "ioport.h" // inb
 #include "memmap.h" // E820_RAM
+#include "pic.h" // eoi_pic2
 
 // Use PS2 System Control port A to set A20 enable
 static inline u8
@@ -368,6 +369,16 @@
     BX_PANIC("NMI Handler called\n");
 }
 
+void
+mathcp_setup()
+{
+    dprintf(3, "math cp init\n");
+    // 80x87 coprocessor installed
+    SETBITS_BDA(equipment_list_flags, 0x02);
+    // Enable IRQ13 (handle_75)
+    unmask_pic2(PIC2_IRQ13);
+}
+
 // INT 75 - IRQ13 - MATH COPROCESSOR EXCEPTION
 void VISIBLE16
 handle_75()
@@ -377,7 +388,7 @@
     // clear irq13
     outb(0, PORT_MATH_CLEAR);
     // clear interrupt
-    eoi_both_pics();
+    eoi_pic2();
     // legacy nmi call
     struct bregs br;
     memset(&br, 0, sizeof(br));