Show enabled locations with disabled breakpoint parent as "y-"

Currently, breakpoint locations that are enabled while their parent
breakpoint is disabled are displayed with "y" in the Enb colum of
"info breakpoints":

 (gdb) info breakpoints
 Num     Type           Disp Enb Address            What
 1       breakpoint     keep n   <MULTIPLE>
 1.1                         y   0x00000000000011b6 in ...
 1.2                         y   0x00000000000011c2 in ...
 1.3                         n   0x00000000000011ce in ...

Such locations won't trigger a break, so to avoid confusion, show "y-"
instead.  For example:

 (gdb) info breakpoints
 Num     Type           Disp Enb Address            What
 1       breakpoint     keep n   <MULTIPLE>
 1.1                         y-  0x00000000000011b6 in ...
 1.2                         y-  0x00000000000011c2 in ...
 1.3                         n   0x00000000000011ce in ...

The "-" sign is inspired on how the TUI represents breakpoints on the
left side of the source window, with "b-" for a disabled breakpoint.

Change-Id: I9952313743c51bf21b4b380c72360ef7d4396a09