config: define and document exit codes

config: define and document exit codes

The return codes of git_config_set() and friends are magic numbers right
in the source. #define them in cache.h where the functions are declared,
and use the constants in the source.

Also, mention the resulting exit codes of "git config" in its man page
(and complete the list).

Signed-off-by: Michael J Gruber <redacted>
Signed-off-by: Junio C Hamano <redacted>