Add support for remapping prefix key

Make it possible to remap the prefix key (default: ctrl-t) by setting
the prefix-ctrl-key variable in the configuration file.

Allowed values are in the range a..z.

Example, to set the prefix key to ctrl-a simply do:

prefix-ctrl-key = a

src/misc.h

@@ -26,3 +26,4 @@
 char * current_time(void);
 void delay(long ms);
 long string_to_long(char *string);
+int ctrl_key_code(unsigned char key);