build: Assume at least C89 and POSIX.1-2001
build: Move obstack check to the functions section
build: Move each warning flag setting in CFLAGS to its own line
doc: Update TODO
doc: Move TODO to the top source directory

This is the usual place where TODO files are found, and it is where
automake expectes it to be to pick it up automatically.
man: Fix formatting
man: Say GnuPG instead of GPG for format keyrings
man: Add references to debsigs(1) and gpg(1)