Fix warnings about shadowed variables, and use -Wshadow in maintainer mode.

Fix warnings about shadowed variables, and use -Wshadow in maintainer mode.

Inspired by a bug in modmcd.c's check_alias_args() spotted by the eagle
eyes of gix.

(I will omit list the function-by-function change log -- most of it is very
mechanical, and involves renaming paramaters and local variables that used
to shadow standard Unix functions.)