Style URLs in GDB output

Style URLs in GDB output

I noticed that GDB will display URLs in a few spots.  This changes
them to be styled.  Originally I thought I'd introduce a new "url"
style, but there aren't many places to use this, so I just reused
filename styling instead.  This patch also changes the debuginfod URL
list to be printed one URL per line.  I think this is probably a bit
easier to read.