make diags location logging more succinct

proxy.config.diags.show_location is a really useful setting to dhow the source location of a debug diagnostic. Unfortunately it logs the full path, making the log lines fairly illegible.

Let's just log the base file name to make debug diags easier to read. This is the set of filenames which are uses more than once:

trafficserver.git jpeach$ find . -type f -name *.h -o -name *.cc | xargs basename | sort | uniq -c | grep -v 1
7 Inline.cc
2 Main.cc
2 Main.h
2 debug_macros.h
2 gzip.cc
2 load_http_hdr.cc
3 lulu.h
2 remap.cc
2 resources.h

That seems acceptable to me.