mem: Rename memmem module to mem

Currently the 'memmem' module does nothing but provide an implementation of
the memmem() function if it is missing from the standard C library.

However there are other functions (e.g. memrchr()) also missing from some
C library implementations, so rename the module to mem to allow future
inclusion of other functions.

This also updates the rfc822 module - the only existing user of the
memmem module - to use the new name.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>