gdb/testsuite: add links for handled control sequences in lib/tuiterm.exp

gdb/testsuite: add links for handled control sequences in lib/tuiterm.exp

This code can be a bit cryptic for those who don't know terminal control
sequences very well.  This patch adds links for all the handled
sequences, so it's easy to get some doc to follow the code.

I linked to a VT510 manual, because I think it's well formatted and easy
to read.  There's only the repeat sequence (_csi_b) which I haven't
found in it, it looks to be xterm-specific or something.

I also tried to use the sequence names as they are in the manual.

gdb/testsuite/ChangeLog:

	* lib/tuiterm.exp: Add links in comments.

Change-Id: I670b947a238e5e9bcab7c476a20eb3c31cf2909d