Cleans the in-source tree built binary.

[CarlosNihelton]

• All builds failed in LP except AMD64 because the binary was in the source tree.
• That was caused by the clean target only removing the installed copy.

Overhauled the Makefiles to:

• Remove the nuisance of realpath being called with empty args.
• Fix the nslogin clean target.
• Make the nslogin default target be the Release one.

During development or debugging one will have the inconvenience to type make Debug to get DDEBUG defined. Noticed though that the option -g was promoted to CFLAGS to preserve the debug symbols in all situations.

[didrocks]

Confirmed that make clean is now doing the right thing! Thanks for the cleanup. Merging.