We are going to use statically allocated struct ppp_mppe_state through PPP PCB,
removed now useless mppe_alloc() and mppe_free().
Merged mppe_alloc() key copy to mppe_init().
Our PPP stack deals with packet without address and control byte nor 2-byte
protocol field, improved mppe_compress() so we don't have to worry about
them.
mppe_decompress() now takes a pointer to a pbuf pointer and re-use the
passed buffer for MPPE "decompression". Removed sub protocol handling
which can be shared among all decompressors in ppp.c
