gdb: small "maintenance info line-table" readability improvements

gdb: small "maintenance info line-table" readability improvements

 - separate each entry with a newline, to visually separate them
 - style filenames with the filename style
 - print the name of the compunit_symtab

A header now looks like this, with the compunit_symtab name added (and
the coloring, but you can't really see it here):

    objfile: /home/simark/build/babeltrace/src/cli/.libs/babeltrace2 ((struct objfile *) 0x613000005980)
    compunit_symtab: babeltrace2-cfg-cli-args.c ((struct compunit_symtab *) 0x62100da1ed10)
    symtab: /usr/include/glib-2.0/glib/gdatetime.h ((struct symtab *) 0x62100d9ee530)
    linetable: ((struct linetable *) 0x0):

Change-Id: Idc23e10aaa66e2e692adb0a6a74144f72c4fa1c7