TCP profile: simplify the decoding of extracted bits
TCP profile: simplify the decoding of extracted bits

The function d_tcp_decode_bits() decodes the bits extracted from the
ROHC header. This function is large. This makes it difficult to read.

This change splits the function into several smaller, separate
functions. This change makes GNU complexity happier.

didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc