Added me as maintainer of extract-docs.c

I'll write this file.
Add error() enhance warn()

error() is a cousin to warn() in the obvious way. Both got respective
configuration variables: continue_on_error and exit_on_warning.

The rest is random fixes. One to note, though: I've added DEBUG to the
exported variables so that we can keep debugging information (I think).
Add elf_targets under p-lang and lib to rm_targets

Now the upp and test executables is also cleaned up there.
Update copyright years and make extern functions

External functions in headers are now marked __export which implies extern.