gdb: adjust the default place of 'list' to main's prologue

The 'list' command prints around the 'main' function if the current
source location is not set.  The prologue of 'main' is skipped and the
first real line of 'main' is offset by 'lines_to_print - 1'.  This is
incorrect, the location should be defaulted to main's prologue without
applying offsets (similar to 'list main').  Printing around the selected
line is then done in 'list_around_line'.

The patch also fixes an issue if the list command is used before the
program is started.  For example, with the following code:

26 static void attribute ((used)) ambiguous_fun (void) {}
27
28 static int attribute ((used)) ambiguous_var;
29
30
31
32
33
34
35
36
37
38 int
39 main (void)
40 {
41   return 0;
42 }

GDB offsets the relevant line by 'lines_to_print - 1' and then by another
'lines_to_print / 2' and prints:

(gdb) list
27
28 static int attribute ((used)) ambiguous_var;
29
30
31
32
33
34
35
36

With this patch, GDB correctly prints:

37
38      int
39      main (void)
40      {
41        return 0;
42      }

Approved-By: Andrew Burgess <aburgess@redhat.com>