Clean up code in inc.c

Clean up code in inc.c

Clean up the confusing use of a variable 'i' in inc.c.
The case where it was being used as 'an error occurred
which means we should not truncate the mailspool' is now
called 'incerr' and merged with the equivalent 'p' variable
used in the POP-specific code. Cases where it was being
used as a random temporary are still called 'i' but now
declared with a much more restricted scope.
This fixes a compilation warning for some compilers.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>