Use a 16-bit comparison for "pppoes NUM".

Use a 16-bit comparison for "pppoes NUM".

gen_pppoes() compares only the low 16 bits of a [32-bit] word, which
does not entirely match the problem space.  Eliminate the bitwise AND
instruction by comparing a [16-bit] half-word with the same 16 bits,
this produces simpler bytecode (thus the corresponding accept test
changes) with an equivalent behaviour (thus no apply tests change).
While at it, add a comment with the packet header diagram and use a
standard named constant for the maximum session ID value.